Field-effect transistor and method for producing field-effect transistor

ABSTRACT

To provide a field-effect transistor, containing: a gate electrode configured to apply gate voltage; a source electrode and a drain electrode, both of which are configured to take out electric current; an active layer formed of a n-type oxide semiconductor, provided in contact with the source electrode and the drain electrode; and a gate insulating layer provided between the gate electrode and the active layer, wherein work function of the source electrode and drain electrode is 4.90 eV or greater, and wherein an electron carrier density of the n-type oxide semiconductor is 4.0×1017 cm−3 or greater.

TECHNICAL FIELD

The present invention relates to a field-effect transistor, and a methodfor producing the field-effect transistor.

BACKGROUND ART

Developments of an active matrix display device (e.g., a liquid crystaldisplay device, a luminescent display device, and an electrophoreticdisplay device), in which a switching element formed of a thin filmtransistor (TFT) that is a field-effect transistor (FET) is provided perdisplay pixel arranged in the formed matrix have been recently activelyconducted.

In these developments, focused is a technology that a TFT is producedusing an oxide semiconductor film, which has high carrier mobility andgive less variation between elements, in the channel formation region ofthe TFT, and the TFT is applied for an electronic device, or an opticdevice. For example, disclosed is the FET using zinc oxide (ZnO), In₂O₃,or In—Ga—Zn—O, as an oxide semiconductor film (see, for example, PTL 1).

As for a field-effect transistor used for a device, demanded is the one,which has high field-effect mobility, high on/off ratio, and a smallabsolute value of turn-on voltage.

A display device, which display in a large area, has a problem of asignal delay from lines to channels of TFTs due to resistance. As for amaterial of an electrode of the TFT, therefore, demanded is use of amaterial having low specific resistance.

When a display device using TFT is produced, a display element islaminated on top of the formed TFT. In the process performed after theformation of the TFT, therefore, a heating treatment, or an oxidizingatmosphere treatment is performed. For this reason, it is desirable touse an electrode material that does not cause deterioration through theheat treatment or oxidizing atmosphere treatment performed after theformation of the TFT.

PTL 1 discloses that a range of a carrier density of an oxidesemiconductor film (e.g., an IGZO film) suitable as a channel of asemiconductive layer is 1×10¹¹ cm⁻³ or greater but lower than 1×10¹⁸cm⁻³, preferably 1×10¹⁴ cm⁻³ or greater but lower than 1×10¹⁷ cm⁻³, morepreferably 1×10¹⁵ cm⁻³ or greater but lower than 1×10¹⁶ cm⁻³.

This is because large electric current may passed through between asource electrode and a drain electrode with no gate voltage of atransistor applied when an oxide the electron carrier density of whichis 1×10¹⁸ cm⁻³ or greater is used as a channel of TFT, and the TFT maybecome a normally-on TFT. In order to produce a normally-off TFTapplicable for an image display device, such as a luminescent device, ithas been widely known that an oxide having an electron carrier densityof lower than 1×10¹⁸ cm⁻³ is used as a channel of the TFT.

Proposed preferable carrier density of an oxide semiconductor layer(IGZO) used for a channel is less than 1×10¹⁷ cm⁻³ (see, for example,PTL 2). This is because a resulting thin film transistor may become anormally-on type if the carrier density is greater than theaforementioned range. Moreover, disclosed is a method where an oxideelectroconductive material having higher carrier density than an oxidesemiconductor layer as a buffer layer between a source-drain electrodelayer and the oxide semiconductor layer, for the purpose of reducingcontact resistance between the oxide semiconductor layer, and thesource-drain electrode layer. This is because contact resistance maybecome large, when a material having low specific resistance (e.g.gold), which is desirable as lines or electrodes, is used as a sourceelectrode and a drain electrode. In the case where a buffer layer is notprovided, a metal having low work function, such as Al, Mo, and Ti, istypically used as an electrode in a TFT using a n-type oxidesemiconductor as an active layer, in order to improve electric contactbetween the active layer and the source and drain electrodes. However,the metal having low work function has a problem that the metal isoxidized during a heat treatment or oxidizing atmosphere treatmentperformed after the formation of a field-effect transistor to increase aspecific resistance.

Moreover, disclosed is to obtain a field-effect transistor, which hashigh mobility and reliability, by controlling an electron carrierdensity n (cm⁻³) in the range of 10¹⁸<n<10²⁰ (see, for example, PTL 3).In the disclosed method, a structure composed of titanium (Ti)/gold(Au)/titanium (Ti) is used as source and drain electrodes.

Moreover, disclosed is to obtain a thin film transistor having a highfield-effect mobility by controlling an electron carrier density n(cm⁻³) in the range of n≤5×10¹⁸ (see, for example, PTL 4). In thedisclosed method, gold (Au) formed into a film by sputtering is used assource and drain electrodes.

Moreover, disclosed is a method, in which a semiconductor layer, and anelectrode layer are formed by coating (see, for example, PTL 5). As forthe method for forming the semiconductor layer of TFT using oxidesemiconductor, or source-drain electrode layer, vacuum deposition, orsputtering is commonly used. In order to perform any of these methods,however, there is a problem that a complicated and expensive device isnecessary. Moreover, another problem is that it is difficult to form athin film of a large area. Accordingly, a method for forming asemiconductor layer or an electrode layer through coating is expected asa method that enables to form a film of a large area with a simplemanner.

A n-type oxide semiconductor tends to exhibits higher mobility, as anelectron carrier density is higher. Therefore, there is a possibilitythat higher on-current is attained, as an electron carrier density ishigher, when the n-type oxide semiconductor is used for a field-effecttransistor.

In the conventional art, however, a field-effect transistor, which has asource electrode and a drain electrode having high resistance to heattreatment and oxidizing atmosphere treatment performed after theformation of the field-effect transistor, and having low specificresistance, does not require a buffer layer, has high field-effectmobility, has high an on/off ratio, and has a small absolute value ofturn-on voltage, has not been provided even in the case where theelectron carrier density of the n-type oxide semiconductor is high.

Accordingly, there is currently a need for a field-effect transistor,which has a source electrode and a drain electrode having highresistance to heat treatment and oxidizing atmosphere treatmentperformed after the formation of the field-effect transistor, and havinglow specific resistance, does not require a buffer layer, has highfield-effect mobility, has high an on/off ratio, and has a smallabsolute value of turn-on voltage, even in the case where the electroncarrier density of the n-type oxide semiconductor is high.

CITATION LIST Patent Literature

-   PTL 1: Japanese Patent (JP-B) No. 5118811-   PTL 2: Japanese Patent Application Laid-Open (JP-A) No. 2010-62546-   PTL 3: JP-A No. 2011-103402-   PTL 4: JP-A No. 2013-4555-   PTL 5: JP-A No. 2010-283190

SUMMARY OF INVENTION Technical Problem

The present invention aims to solve the aforementioned various problemsin the art, and to achieve the following object. Specifically, theobject of the present invention is to provide a field-effect transistor,which has a source electrode and drain electrode that are highlyresistant to a heating process and oxidizing atmosphere processingperformed after formation of a field-effect transistor, even when acarrier density of a n-type oxide semiconductor is high, and have lowspecific resistance, and which does not require a buffer layer, whichhas high a field-effect mobility, high on/off ratio, and a smallabsolute value of turn-on voltage, even in the case where the electroncarrier density of the n-type oxide semiconductor is high.

Solution to Problem

The means for solving the aforementioned problems are as follows.

The field-effect transistor of the present invention contains:

a gate electrode configured to apply gate voltage;

a source electrode and a drain electrode, both of which are configuredto take out electric current;

an active layer formed of a n-type oxide semiconductor, provided incontact with the source electrode and the drain electrode; and

a gate insulating layer provided between the gate electrode and theactive layer,

wherein work function of the source electrode and the drain electrode is4.90 eV or greater, and

wherein an electron carrier density of the n-type oxide semiconductor is4.0×10¹⁷ cm⁻³ or greater.

Advantageous Effects of Invention

The present invention can solve the aforementioned various problems inthe art, and can provide a field-effect transistor, which has a sourceelectrode and drain electrode that are highly resistant to a heatingprocess and oxidizing atmosphere processing performed after formation ofa field-effect transistor, even when a carrier density of a n-type oxidesemiconductor is high, and have low specific resistance, and which doesnot require a buffer layer, which has high a field-effect mobility, highon/off ratio, and a small absolute value of turn-on voltage, even in thecase where the electron carrier density of the n-type oxidesemiconductor is high.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating one example of abottom gate/bottom contact field-effect transistor.

FIG. 2 is a schematic cross-sectional view illustrating one example of abottom gate/top contact field-effect transistor.

FIG. 3 is a schematic cross-sectional view illustrating one example of atop gate/bottom contact field-effect transistor.

FIG. 4 is a schematic cross-sectional view illustrating one example of atop gate/top contact field-effect transistor.

FIG. 5A is a schematic cross-sectional view illustrating one example ofa method of producing a field-effect transistor according to the presentinvention (part 1).

FIG. 5B is a schematic cross-sectional view illustrating one example ofa method of producing a field-effect transistor according to the presentinvention (part 2).

FIG. 5C is a schematic cross-sectional view illustrating one example ofa method of producing a field-effect transistor according to the presentinvention (part 3).

FIG. 5D is a schematic cross-sectional view illustrating one example ofa method of producing a field-effect transistor according to the presentinvention (part 4).

FIG. 6 is a diagram depicting current-voltage characteristics of thefield-effect transistor of Comparative Example 1 with the carrierdensity of 5.9×10¹⁵ cm⁻³.

FIG. 7 is a diagram depicting current-voltage characteristics of thefield-effect transistor of Example 1 with the carrier density of1.0×10¹⁸ cm⁻³.

FIG. 8 is a diagram depicting current-voltage characteristics of thefield-effect transistor using a Mg—In based oxide semiconductor thinfilm (electron carrier density: 5.9×10¹⁵ cm⁻³) as a n-type oxidesemiconductor, and Al as a source electrode and drain electrode.

FIG. 9 is a diagram depicting current-voltage characteristics of thefield-effect transistor using a Mg—In based oxide semiconductor thinfilm (electron carrier density: 1.0×10¹⁸ cm⁻³) as a n-type oxidesemiconductor, and Al as a source electrode and drain electrode.

FIG. 10 is a schematic diagram illustrating one example of a televisiondevice as the system of the present invention.

FIG. 11 is a diagram (part 1) for explaining the image display device ofFIG. 10.

FIG. 12 is a diagram (part 2) for explaining the image display device ofFIG. 10.

FIG. 13 is a diagram (part 3) for explaining the image display device ofFIG. 10.

FIG. 14 is a diagram for explaining one example of the display elementof the present invention.

FIG. 15 is a schematic diagram depicting one example of the arrangementof an organic EL element and a field-effect transistor in a displayelement.

FIG. 16 is a schematic diagram depicting another example of thearrangement of an organic EL element and a field-effect transistor in adisplay element.

FIG. 17 is a schematic diagram illustrating one example of an organic ELelement.

FIG. 18 is a diagram for explaining a display control device.

FIG. 19 is a diagram for explaining a liquid crystal display.

FIG. 20 is a diagram for explaining the display element of FIG. 19.

FIG. 21 is a diagram depicting current-voltage characteristics of thefield-effect transistor of Example 18 with the carrier density of5.2×10¹⁸ cm⁻³.

FIG. 22 is a diagram depicting current-voltage characteristics of thefield-effect transistor of Example 18 with the carrier density of5.2×10¹⁸ cm⁻³.

FIG. 23 is a diagram depicting current-voltage characteristics of thefield-effect transistor of Comparative Example 3 with the carrierdensity of 5.7×10¹⁵ cm⁻³.

FIG. 24 is a diagram depicting current-voltage characteristics of thefield-effect transistor of Comparative Example 3 with the carrierdensity of 5.7×10¹⁵ cm⁻³.

DESCRIPTION OF EMBODIMENTS Field-Effect Transistor

The field-effect transistor of the present invention contains at least agate electrode, a source electrode, a drain electrode, an active layer,and a gate insulating layer, and may further contain other members,according to the necessity.

The field-effect transistor of the present invention can be produced,for example, by the method for producing the field-effect transistoraccording to the present invention.

In the aforementioned JP-B No. 5118811 and JP-A No. 2010-62546,disclosed is a technique for realizing a normally-off transistoroperation by controlling an electron carrier density of an oxidesemiconductor layer to a range of 1×10¹¹ cm⁻³ to 1×10¹⁸ cm⁻³. In JP-BNo. 5118811, gold is used for source-drain terminals, but an In—Ga—Zn—Oamorphous film having a large electroconductivity is introduced betweenan oxide semiconductor layer that will be a channel, and the gold.

As a result of the researches conducted by the present inventors, it hasbeen confirmed that sufficient transistor operation cannot be attainedwith a TFT, in which an active layer formed of an oxide semiconductorfilm, the carrier density of which is in the range of 1×10¹⁵ cm⁻³ orgreater but less than 1×10¹⁶ cm⁻³ which is disclosed as a preferablerange in JP-B No. 5118811, and source and drain electrodes formed ofgold are directly connected. The reason for this is as follows. In thecase where a metal having high work function, such as gold, is broughtinto contact with a n-type oxide semiconductor having low work function,Schottky barrier junction is formed at a contact interface, and as aresult, it is considered that resistance becomes high.

In JP-A No. 2011-103402, it is disclosed that a field-effect transistorhaving high mobility and reliability can be attained by controlling theelectron carrier density n (cm⁻³) in the range of 10¹⁸<n<10²⁰, and usingtitanium (Ti)/gold (Au)/titanium (Ti) for source and drain electrodes.In this transistor, the plane at the side of the electrode in thecontact area between the active layer of the oxide semiconductor, andthe source and drain electrodes is titanium.

As a result of the researches conducted by the present inventors,however, it has been confirmed that a high on/off ration cannot beattained with a field-effect transistor using an oxide semiconductorhaving the electron carrier density of 1×10¹⁸ cm⁻³ or greater, andtitanium for the source and drain electrodes. Since the metal having lowwork function, i.e., titanium, is brought into contact with the oxidesemiconductor, electric contact between the oxide semiconductor and theelectrodes is excellent. However, a high on/off ratio cannot be attainedfrom the reasons disclosed in JP-B No. 5118811, and JP-A No. 2010-62546.

In JP-A No. 2010-283190, disclosed is a TFT, in which a source electrodeand a drain electrode are formed by applying Ag nano particles throughan inkjet method.

As a result of the researches conducted by the present inventors,however, a TFT using an oxide semiconductor film having the carrierdensity in the range of 1×10¹⁵ cm⁻³ or greater but less than 1×10¹⁶cm⁻³, which is disclosed as a more preferable range, and source anddrain electrodes formed by coating Ag nano particles has poor electricconduct between the oxide semiconductor and the electrodes, and cannotattain high field-effect mobility (may be referred as “carrier mobility”hereinafter). A thin film, which is formed by coating followed bybaking, and has low specific resistance, is typically silver (Ag) orgold (Au). There has not been yet a technology where a metal material,such as Al, Mo, and Ti, is formed into an ink, and is applied as theink, followed by baking, to thereby form a thin film.

Other than the metal material, as for an electroconductive material thatcan be formed into a film by coating, typically known is, for example,tin-doped indium oxide (ITO). However, the specific resistance of theITO film formed by coating is higher than that of the film formed by avacuum process. In a display device, in which a display of a large areais presented, a delay of a signal due to resistance from lines tochannel of the TFT becomes significant. Therefore, it is not sufficientto apply as an electrode material of TFT.

The present inventors have researched as follow based on the result ofthe researches associated with the conventional art.

The present inventors produced a field-effect transistor, where Mg—Inbased oxide semiconductor was used as a n-type oxide semiconductor, andAl was used as a source electrode and a drain electrode, and conductedan experiment for confirming the transistor characteristics. Theelectron carrier density of the Mg—In based oxide semiconductor wasmeasured by the below-mentioned hall effect measurement device, and theresult was 5.9×10¹⁵ cm⁻³.

FIG. 8 is a diagram depicting current-voltage characteristics of abottom gate/top contact field-effect transistor, and shows arelationship between the gate voltage Vg and the source-drain electriccurrent Ids with the source-drain voltage Vds of 20 V. The field-effectmobility calculated in the saturated region was 4.6 cm²/Vs. Moreover, aratio (on/off ratio) of the source-drain electric current in theon-state (e.g., Vg=20V) of the transistor to that in the off-state(e.g., Vg=−20 V) was 5.9×10⁷. Moreover, the gate voltage (turn-onvoltage) at which the source-drain electric current was turned toincrease was Vg=−2 V.

FIG. 9 depicts the results of the measurements of the current-voltagecharacteristics of the produced bottom gate/top contact field-effecttransistor where the electron carrier density of the Mg—In based oxidesemiconductor is 1.0×10¹⁸ cm⁻³. When the gate voltage was in the rangeof −30V to +30V, the transistor was not turned into a clear off state.The ratio of the source-drain current with Vg=20 V to that with Vg=−20 Vwas 3.3×10¹.

In the case where the electron carrier density of the oxidesemiconductor is 1×10¹⁵ cm⁻³ to 1×10¹⁶ cm⁻³, and the metal having lowwork function is used as source and drain electrodes, as describedabove, transistor characteristics that the field-effect mobility ishigh, the on/off ratio is high, and the absolute value of the turn-onvoltage is small can be attained.

When the electron carrier density is 1.0×10¹⁸ cm⁻³ or greater, on theother hand, the transistor characteristics that the on/off ratio issmall, and the absolute value of the turn-on voltage is large areobtained.

The transistor having the performance as depicted in FIG. 8 can besuitably used for an active matrix display device in view of theperformance, but the Al electrode is deteriorated when a heat treatmentand oxidizing atmosphere treatment are performed after the formation ofthe transistor, and as a result, the deterioration of the transistorperformance is confirmed.

Therefore, the present inventors produced a field-effect transistorusing gold as source and drain electrodes, and conducted an experimentto confirm the transistor characteristics of the produced field-effecttransistor.

FIG. 6 (the case where the carrier density is 5.9×10¹⁵ cm⁻³ inComparative Example 1) and FIG. 7 (the case where the carrier density is1.0×10¹⁸ cm⁻³ in Example 1) are diagrams depicting current-voltagecharacteristics of the field-effect transistor using gold as the sourceand drain electrode, with the electron carrier density of the Mg—Inbased oxide semiconductor being 5.9×10¹⁵ cm⁻³, and 1.0×10¹⁸ cm⁻³respectively.

It can be understood from FIG. 6 that the on-current is low, i.e., about1×10⁻⁹ A, and the transistor hardly operates, when the electron carrierdensity is in the range of 1×10¹⁵ cm⁻³ to 1×10¹⁶ cm⁻³, which isconsidered to be more suitably used as a transistor using a n-type oxidesemiconductor, and gold is used as source and drain electrode. This isbecause Schottky barrier junction is formed at a plane where the goldhaving a relatively high work function and the n-type oxidesemiconductor are in contact, and thus contact resistance becomes large.

When gold is used as source and drain electrode with the electroncarrier density of 1×10¹⁸ cm⁻³ or greater, which has been conventionallyconsidered as a range that is not suitably used for a transistor using an-type oxide semiconductor, on the other hand, it has been found fromFIG. 7 that the transistor characteristics that the field-effectmobility is high, i.e., 5.6 cm²/Vs, the ratio (on/off ratio) of thesource-drain current with Vg=20 V to that with Vg=−20 V is high, i.e.,4.7×10⁸, and the absolute value of the turn-on voltage is small, i.e.,Vg=1.0 V.

The transistors depicted in FIGS. 6 to 9 exhibited desirableperformances due to a combination with the electron carrier density ofthe n-type oxide semiconductor and the gold, as the material of thesource and drain electrode and the electron carrier density are varied.

Furthermore, the present inventors have conducted an experiment todetermine the work function of a metal formed by vacuum deposition, suchas sputtering, and vacuum vapor deposition, and the work function of ametal formed by applying nanoparticles. As a result, it has been foundthat the work function of the metal thin film indicated a differentvalue depending on the film forming method, and the metal formed byapplying nanoparticles has a higher work function than that of the metalformed by vacuum deposition, even through they are identical metal.

For example, the present inventors have conducted an experiment todetermine work functions of gold (Au), and silver (Ag) formed by vacuumdeposition, and work functions of gold (Au), and silver (Ag) formed byapplying nanoparticles or an organometallic compound. As a result, thework functions of the gold (Au), and silver (Ag) formed by vacuumdeposition are respectively 4.70 eV to 4.79 eV, and 4.80 eV to 4.83 eV.On the other hand, the work functions of the gold (Au), and silver (Ag)formed by applying nanoparticles or an organometallic compound arerespectively 4.90 eV to 5.10 eV, and 5.05 eV to 5.30 V. It has beenfound that the metal formed by applying nanoparticles or anorganometallic compound has a higher work function than the metal formedby vacuum deposition.

As a result of further researches conducted based on the aforementionedresults, it has been found that a field-effect transistor, which has asource electrode and a drain electrode having high resistance to a heattreatment and oxidizing atmosphere treatment performed after theformation of the field-effect transistor, and having low specificresistance, does not require a buffer layer, has a high electric-fieldmobility, high on/off ratio, and a small absolute value of the turn-onvoltage, can be provided when the work function of the source electrodeand drain electrode is 4.90 eV or greater, and the electron carrierdensity of the n-type oxide semiconductor is 4.0×10¹⁷ cm⁻³ or greater.

Moreover, it has been found that a field-effect transistor, in which asource electrode and a drain electrode are both formed by applying nanoparticles or an organometallic compound (e.g., Au, Ag, Ag/Pd, and Pt)has a high field-effect mobility, high on/off ratio, and a smallabsolute value of the turn-on voltage, even when the electron carrierdensity of the n-type oxide semiconductor is greater than 5.0×10¹⁸ cm⁻³.

<Gate Electrode>

The gate electrode is appropriately selected depending on the intendedpurpose without any limitation, provided that it is an electrodeconfigured to apply gate voltage.

A material of the gate electrode is appropriately selected depending onthe intended purpose without any limitation, and examples thereofinclude: a metal, such as platinum, palladium, gold, silver, copper,zinc, aluminum, nickel, chromium, tantalum, molybdenum, and titanium;alloys of the aforementioned metals; and a mixture of the aforementionedmetals. Moreover, further examples thereof include: an electroconductiveoxide, such as indium oxide, zinc oxide, tin oxide, gallium oxide,niobium oxide, tin (Sn)-doped In₂O₃ (ITO), gallium (Ga)-doped ZnO,aluminum (Al)-doped ZnO, and antimony (Sb)-doped SnO₂; a compositecompound thereof; and a mixture thereof.

The average thickness of the gate electrode is appropriately selecteddepending on the intended purpose without any limitation, but theaverage thickness thereof is preferably 10 nm to 200 nm, more preferably50 nm to 100 nm.

<Gate Insulating Layer>

The gate insulating layer is appropriately selected depending on theintended purpose without any limitation, provided that it is aninsulating layer formed between the gate electrode and the active layer.

A material of the gate insulating layer is appropriately selecteddepending on the intended purpose without any limitation, and examplesthereof include an inorganic insulating material, and an organicinsulating material.

Examples of the inorganic insulating material include silicon oxide,aluminum oxide, tantalum oxide, titanium oxide, yttrium oxide, lanthanumoxide, hafnium oxide, zirconium oxide, silicon nitride, aluminumnitride, and a mixture thereof.

Examples of the organic insulating material include polyimide,polyamide, polyacrylate, polyvinyl alcohol, and a novolak resin.

The average thickness of the gate insulating layer is appropriatelyselected depending on the intended purpose without any limitation, butthe average thickness thereof is preferably 50 nm to 1,000 nm, morepreferably 100 nm to 500 nm.

<Source Electrode and Drain Electrode>

The source electrode and the drain electrode are electrodes configuredto take out electric current.

The work function of the source electrode and the drain electrode is4.90 eV or greater.

As the work function of the source electrode and the drain electrode is4.90 eV or greater, the resistance of the source electrode and drainelectrode to the heat treatment or oxidizing atmosphere treatmentperformed after the formation of the field-effect transistor becomeshigh, and the specific resistance of the source electrode and the drainelectrode is maintained low.

A material of the source electrode and the drain electrode is preferablya metal, an alloy, or both. Here, the phrase “a metal, an alloy, orboth” does not exclude to contain impurities to a degree that does notadversely affect the work function and the specific resistance.

Examples of the metal include gold, silver, palladium, platinum, nickel,iridium, and rhodium.

The alloy includes those exhibiting metallic conductivity, and composedof a plurality of metal elements, or a combination of a metal elementand a non-metal element, and examples thereof include a solid solution,eutectic crystal, and an intermetallic compound.

The alloy is preferably an alloy of a plurality of metals selected fromthe group consisting of gold, silver, palladium, platinum, nickel,iridium, and rhodium.

The source electrode and the drain electrode may be each formed byelectrically connecting a plurality of the metals, a plurality of thealloys, or a combination of a single or plurality of the metal and asingle or plurality of the alloys.

The source electrode and the drain electrode are preferably formed bybaking metal particles, alloy particles, an organometallic compound, orany combination thereof.

Examples of the metal particles include gold particles, silverparticles, palladium particles, platinum particles, nickel particles,iridium particles, and rhodium particles.

Examples of the alloy particles include particles formed of any of theaforementioned alloys. For example, the alloy particles aresilver-palladium alloy particles.

The alloy particles may have a core-shell structure, or layeredstructure, formed of a plurality of the aforementioned metals.

The metal particles and the alloy particles may contain a surfactant ora dispersing agent for improving dispersibility of the particles in asolvent, as long as it does not adversely affect the obtainable effectof the present invention. Moreover, the metal particles and the alloyparticles may contain a protective agent for improving storage stabilitywhen the particles are dispersed in a solvent.

The source electrode and the drain electrode may be each formed byelectrically connecting a plurality of the metal particles, a pluralityof the alloy particles, or a combination of a single or plurality of themetal particles, and a single or plurality of the alloy particles.

The organometallic compound is appropriately selected depending on theintended purpose without any limitation, provided that it is a compoundcontaining the metal and an organic group. The term “organometalliccompound” indicates, in a narrow sense, a compound containing ametal-carbon bond. In the present specification, however, in addition tothe metal-carbon bond, the term “organometallic compound” also includesa compound, in which the metal and an organic group are bonded through acovalent bond, an ionic bond, or a coordinate bond.

Examples of the metal-carbon bond include a metal-carbonyl bond, ametal-alkyl bond, and a metal-olefin bond. Examples of the organic groupbonded to the metal through the metal-carbon bond include a carbonylgroup, an alkyl group, an alkenyl group, and an alkynyl group.

Examples of the organic group bonded to the metal through the covalentbond include an alkoxy group.

Examples of the organic group bonded to the metal through the ionic bondinclude organic acid, such as carboxylic acid, and octylic acid.

Examples of the organic group bonded to the metal through the coordinatebond include an acetylacetonato group, and a thiolate group.

Examples of the organometallic compound include a metal acetylidecompound, metal alkoxide, metal carboxylate, and a metal thiolatecomplex.

The source electrode and the drain electrode are preferably formed byapplying a coating liquid containing metal particles, alloy particles,an organometallic compound, or any combination thereof in a dropletejecting system, and baking the metal particles, the alloy particles,the organometallic compound, or any combination thereof. Examples of thedroplet ejecting system include an inkjet system.

—Coating Liquid—

The coating liquid is appropriately selected depending on the intendedpurpose without any limitation, provided that the coating liquidcontains the metal particles, the alloy particles, the organometalliccompound, or any combination thereof. The coating liquid is preferably acoating liquid containing gold particles, silver particles,silver-palladium alloy particles, palladium particles, platinumparticles, nickel particles, iridium particles, rhodium particles, theorganometallic compound, or any combination thereof.

Shapes of the metal particles and the alloy particles are appropriatelyselected depending on the intended purpose without any limitation, andexamples thereof include spherical shapes, ellipsoidal shapes, andpolyhedral shapes. Among them, spherical shapes are preferable. Notethat, the spherical shapes are not limited to spheres.

The average particle diameter of the metal particles or alloy particlesis appropriately selected depending on the intended purpose without anylimitation, but the upper limit thereof is preferably 1 μm or smaller,more preferably 500 nm or smaller, and particularly preferably 100 nm orsmaller. The lower limit of the average particle diameter is preferably1 nm or greater, more preferably 5 nm or greater.

It is known that metal nano particles can be typically sintered attemperature that is significantly lower than the melting point of thebulk metal, when the average particle diameter thereof is severalnanometers to several ten nanometers. An effect for lowering bakingtemperature is expected by setting the average particle diameter to therange of 1 nm to 10 nm. In the case where the source electrode and thedrain electrode are formed using a droplet ejecting system, such asinkjet, it is considered that the small average particle size cancontribute to prevention of clogging of a nozzle.

The coating liquid may contain an organic solvent. Examples of theorganic solvent include: hydrocarbon, such as tetradecane; cyclichydrocarbon, such as cyclohexane, and cyclododecene; aromatichydrocarbon, such as toluene, xylene, and mesitylene; glycol ether, suchas diethylene glycol monoethyl ether; monoalcohol ether, such asethylene glycol monomethyl ether; polyhydric alcohol, such as ethyleneglycol, and propylene glycol; and monoalcohol, such as butanol. Thesemay be used alone, or in combination.

The work function of the source electrode and the drain electrode is4.90 eV or greater, preferably 5.00 eV or greater. When the workfunction is less than 4.90 eV, it is difficult to stably realize a highon/off ratio and a small absolute value of the turn-on voltage with ahigh electron carrier density. The upper limit of the work function isappropriately selected depending on the intended purpose without anylimitation, but the work function is preferably 6.00 eV or less.

A measurement of the work function can be carried out, for example, bymeans of a photoelectron spectrometer in air AC-2 (manufactured by RIKENKEIKI Co., Ltd.).

The average thickness of the source electrode and the drain electrode isappropriately selected depending on the intended purpose without anylimitation, but the average thickness thereof is preferably 10 nm to 500nm, more preferably 50 nm to 200 nm.

<Active Layer>

The active layer is provided in contact with the source electrode andthe drain electrode.

The active layer is formed of a n-type oxide semiconductor.

The electron carrier density of the n-type oxide semiconductor is4.0×10¹⁷ cm⁻³ or greater, preferably 1.0×10¹⁸ cm⁻³ or greater, morepreferably 5.1×10¹⁸ cm⁻³ or greater, and particularly preferably7.5×10¹⁸ cm⁻³ or greater. The upper limit of the electron carrierdensity is appropriately selected depending on the intended purposewithout any limitation, but the electron carrier density is preferably1.0×10²⁰ cm⁻³ or lower, more preferably 5.0×10¹⁹ cm⁻³ or lower.

When the electron carrier density is lower than 4.0×10¹⁷ cm⁻³, aresulting field-effect transistor cannot attain excellent transistorcharacteristics.

When the electron carrier density is greater than 1.0×10²⁰ cm⁻³, it isdifficult to attain a high on/off ratio, and a small absolute value ofturn-on voltage.

The electron carrier density can be measured, for example, by means of ahall effect measurement device.

The hall effect is a phenomenon that a electromotive force is generatedin the direction transverse to both electric current and a magneticfield as the magnetic field vertical to the electric current is applied,and is used to mainly identify a carrier density, mobility, and acarrier type of a semiconductor.

Examples of the hall effect measurement device include a specificresistance/hall effect measurement system ResiTest8300 (manufactured byTOYO Corporation).

The n-type oxide semiconductor is appropriately selected depending onthe intended purpose without any limitation, provided that it has theelectron carrier density of 4.0×10¹⁷ cm⁻³ or greater. However, then-type oxide semiconductor preferably contains indium, zinc, tin,gallium, titanium, or any combination thereof.

Examples of the n-type oxide semiconductor include ZnO, SnO₂, In₂O₃,TiO₂, and Ga₂O₃. Moreover, an oxide containing a plurality of metals,such as In—Zn based oxide, In—Sn based oxide, In—Ga based oxide, Sn—Znbased oxide, Sn—Ga based oxide, Zn—Ga based oxide, In—Zn—Sn based oxide,In—Ga—Zn based oxide, In—Sn—Ga based oxide, Sn—Ga—Zn based oxide,In—Al—Zn based oxide, Al—Ga—Zn based oxide, Sn—Al—Zn based oxide,In—Hf—Zn based oxide, and In—Al—Ga—Zn based oxide, can be also used.

The n-type oxide semiconductor preferably contains indium, zinc, tin,gallium, titanium, or any combination thereof, and an alkaline earthmetal, as high field-effect mobility is attained, and the electroncarrier density is easily controlled appropriately. It is more preferredthat the n-type oxide semiconductor contain indium and an alkaline earthmetal.

Examples of the alkaline earth metal include beryllium, magnesium,calcium, strontium, barium, and radium.

Indium oxide changes its electron carrier density in the range of about10¹⁸ cm⁻³ to about 10²⁰ cm⁻³ depending on an amount of oxygen defect. Itshould be noted that indium oxide tends to cause oxygen defects, andthere is a case where unintentional oxygen defects may be formed in alater step after formation of an oxide semiconductor film. Formation ofoxide mainly from two metals that are indium, and an alkaline earthmetal, which is more easily bonded to oxygen than indium, isparticularly preferable, because formation of unintentional oxygendefects can be prevented, and an electron carrier density isappropriately controlled, as a composition of the oxide is easilycontrolled.

The electron carrier density of the active layer can be controlled to anappropriate range by elements for constituting the active layer,production process conditions, and a post treatment performed afterforming the active layer.

The average thickness of the active layer is appropriately selecteddepending on the intended purpose without any limitation, but theaverage thickness thereof is preferably 1 nm to 200 nm, more preferably2 nm to 100 nm.

A structure of the field-effect transistor is appropriately selecteddepending on the intended purpose without any limitation, and examplesthereof include a bottom gate/bottom contact type (FIG. 1), a bottomgate/top contact type (FIG. 2), a top gate/bottom contact type (FIG. 3),and a top gate/top contact type (FIG. 4).

In FIGS. 1 to 4, 1 is a base, 2 is a gate electrode, 3 is a gateinsulating layer, 4 is a source electrode, 5 is a drain electrode, and 6is an active layer.

The field-effect transistor does not require a buffer layer, as thecarrier density thereof is 4.0×10¹⁷ cm⁻³ or greater, and thus contactresistance at a contact surface between the oxide semiconductor and thesource electrode and drain electrode does not become large.

The field-effect transistor of the present invention is suitably used asa pixel driving circuit of a liquid crystal display, an organic ELdisplay, or an electrochromic display, or a field-effect transistor fora logic circuit.

(Method for Producing Field-Effect Transistor)

<First Production Method>

The method for forming a field-effect transistor according to thepresent invention (first production method) contains at least a sourceelectrode and drain electrode forming step, and may further containother steps, such as a gate electrode forming step, a gate insulatinglayer forming step, and an active layer forming step, according to thenecessity.

The method for forming the field-effect transistor is a method forforming the field-effect transistor of the present invention.

<<Gate Electrode Forming Step>>

The gate electrode forming step is appropriately selected depending onthe intended purpose without any limitation, provided that it is a stepcontaining forming a gate electrode on the base. Examples thereofinclude: (i) a step containing, after forming a film through sputteringor dip coating, patterning the film through photolithography; and (ii) astep containing directly forming a film of a desired shape through aprinting process, such as inkjet printing, nano imprinting, and gravureprinting.

—Base—

A shape, structure, and size of the base are appropriately selecteddepending on the intended purpose without any limitation.

A material of the base is appropriately selected depending on theintended purpose without any limitation, and examples thereof include aglass base, and a plastic base.

The glass base is appropriately selected depending on the intendedpurpose without any limitation, and examples thereof include non-alkaliglass, and silica glass.

The plastic base is appropriately selected depending on the intendedpurpose without any limitation, and examples thereof includepolycarbonate (PC), polyimide (PI), polyethylene terephthalate (PET),and polyethylene naphthalate (PEN). Note that, a pre-treatment, such asoxygen plasma, UV ozone, and UV radiation washing, is preferablyperformed on the base to clean a surface thereof and to improve adhesionwith another layer.

<<Gate Insulating Layer Forming Step>>

The gate insulating layer forming step is appropriately selecteddepending on the intended purpose without any limitation, provided thatit is a step containing forming a gate insulating layer on the gateelectrode. Examples thereof include: (i) a step containing, afterforming a film through sputtering or dip coating, patterning the filmthrough photolithography; and (ii) a step containing directly forming afilm of a desired shape through a printing process, such as inkjetprinting, nano imprinting, and gravure printing.

<<Source Electrode and Drain Electrode Forming Step>>

The source electrode and drain electrode forming step is a stepcontaining applying a coating liquid containing metal particles, alloyparticles, an organometallic compound, or any combination thereof on thegate insulating layer, and baking the metal particles, the alloyparticles, the organometallic compound, or any combination thereof, tothereby form a source electrode and a drain electrode.

As a result of the source electrode and drain electrode forming step, asource electrode and a drain electrode are formed at least on the gateinsulating layer with a space between the source electrode and the drainelectrode.

A method for the applying is appropriately selected depending on theintended purpose without any limitation, and examples thereof includescreen printing, roller coating, dip coating, spin coating, inkjetprinting, and nano imprinting.

When inkjet printing or nano imprinting is used as the applicationmethod, the coating liquid can be applied at room temperature. Byheating a base to be coated (e.g., the gate insulating layer) to a rangeof about 25° C. to about 50° C., wetting and spreading of the coatingliquid on a surface of the base to be coated can be prevented just afterthe deposition of the coating liquid.

After the applying, drying and baking are preferably carried out.

The drying is appropriately selected depending on the intended purposewithout any limitation, provided that it is performed under theconditions that can remove volatile components in the coating liquid.Note that, it is not necessary to completely remove the volatilecomponent through the drying, as long as the volatile components areremoved to the degree that baking is not adversely affected.

The temperature of the baking is appropriately selected depending on theintended purpose without any limitation, provided that it is equal to orlower than the thermal deformation temperature of a base to be coated.The temperature of the baking is preferably 180° C. to 600° C.

The atmosphere of the baking is appropriately selected depending on theintended purpose without any limitation, and examples thereof includeatmosphere containing oxygen, such as oxygen, and air. Moreover, theatmosphere of the baking may be inert gas, such as nitrogen gas.

The duration of the baking is appropriately selected depending on theintended purpose without any limitation.

<<Active Layer Forming Step>>

The active layer forming step is appropriately selected depending on theintended purpose without any limitation, provided that it is a stepcontaining forming an active layer formed of a n-type oxidesemiconductor at least on the gate insulating layer that will be achannel region. Examples thereof include: physical vapor deposition(physical vapor growth) such as sputtering, and pulsed laser deposition(PLD); chemical vapor deposition, such as plasma CVD; a solution coatingmethod, such as a sol-gel method; and other conventional film formingmethods. Examples of the patterning method of the active layer include amethod using a shadow mask, a method using photolithography, and amethod containing directly forming a film of an desired shape throughprinting or inkjet printing.

As the active layer forming step, and the source electrode and drainelectrode forming step are carried out, the active layer is formedbetween the source electrode and the drain electrode.

When physical vapor deposition, such as sputtering, is used, an amountof oxygen in the n-type oxide semiconductor can be controlled with anamount of oxygen in the film forming atmosphere. In the case whereelectron carriers are generated by oxygen defects of the n-type oxidesemiconductor, the electron carrier density of the n-type oxidesemiconductor can be increased by lowering the oxygen amount of the filmforming atmosphere.

Moreover, the oxygen content in the film can be controlled by carryingout a heating treatment after forming the n-type oxide semiconductor. Inthis case, the electron carrier density of the n-type oxidesemiconductor can be controlled to the predetermined value by adjustingheating temperature, heating duration, heating speed, cooling speed, andatmosphere (fractions of gas, pressure) for heating.

When a wet process, such as a solution coating method, is used, theelectron carrier density in the n-type oxide semiconductor can becontrolled to the predetermined value by adjusting conditions of a heattreatment performed after coating, specifically, baking temperature,baking duration, heating speed, cooling speed, and atmosphere (fractionsof bas, pressure) for baking. Moreover, the electron carrier density canbe controlled by further carrying out a heat treatment after baking.

In the first production method, an order for carrying out the sourceelectrode and drain electrode forming step, and the active layer formingstep is not restricted. The active layer forming step may be performedafter the source electrode and drain electrode forming step.Alternatively, the source electrode and drain electrode forming step maybe performed after the active layer forming step.

When the active layer forming step is performed after the sourceelectrode and drain electrode forming step in the first productionmethod, a bottom gate/bottom contact field-effect transistor can beproduced.

When the source electrode and drain electrode forming step is performedafter the active layer forming step in the first production method, abottom gate/top contact field-effect transistor can be produced.

A production method of a bottom gate/top contact field-effect transistoris explained hereinafter with reference to FIGS. 5A to 5D.

First, an electroconductor film formed of aluminum is formed on a base 1formed of a glass substrate by sputtering, and the formedelectroconductor film is patterned by etching, to thereby form a gateelectrode 2 (FIG. 5A).

Subsequently, a gate insulating layer 3 formed of SiO₂ is formed on thegate electrode 2 and the base 1 by sputtering to cover the gateelectrode 2 (FIG. 5B).

Then, a n-type oxide semiconductor film is formed on the gate insulatinglayer 3 by sputtering, and the formed oxide semiconductor film ispatterned by etching, to thereby form an active layer 6 (FIG. 5C).

Subsequently, a coating liquid containing metal particles, alloyparticles, an organometallic compound, or any combination thereof isapplied by a droplet ejecting system, such as an inkjet system to coverpart of the active layer 6, followed by performing a heat treatment, tothereby form a source electrode 4 and a drain electrode 5 (FIG. 5D).

In the manner as described above, a field-effect transistor is produced.

<Second Production Method>

A method for producing a field-effect transistor according to thepresent invention (second production method) contains at least a sourceelectrode and drain electrode forming step, and may further containother steps according to the necessity.

The method for producing a field-effect transistor (second productionmethod) is a method for producing the field-effect transistor of thepresent invention.

<<Source Electrode and Drain Electrode Forming Step>>

The source electrode and drain electrode forming step is applying acoating liquid containing metal particles, alloy particles, anorganometallic compound, or any combination thereof at least onto abase, and baking the metal particles, the alloy particles, theorganometallic compound, or any combination thereof, to thereby form asource electrode and a drain electrode.

As a result of the source electrode and drain electrode forming step, asource electrode and a drain electrode are formed at least on the basewith a space between the source electrode and the drain electrode.

—Base—

The base is appropriately selected depending on the intended purposewithout any limitation, and examples thereof include those listed as thebase in the first production method.

Examples of the source electrode and drain electrode forming stepinclude methods listed as the source electrode and drain electrodeforming step in the first production method.

<<Active Layer Forming Step>>

The active layer forming step is appropriately selected depending on theintended purpose without any limitation, provided that it containsforming an active layer formed of a n-type oxide semiconductor at leaston the base that will be a channel region.

Examples of the active layer forming step include methods listed as theactive layer forming step in the first production method.

As the active layer forming step, and the source electrode and drainelectrode forming step are carried out, the active layer is formedbetween the source electrode and the drain electrode.

<<Gate Insulating Layer Forming Step>>

The gate insulating layer forming step is appropriately selecteddepending on the intended purpose without any limitation, provided thatis contains forming an gate insulating layer on the active layer.Examples thereof include methods listed as the gate insulating layerforming step in the first production method.

<<Gate Electrode Forming Step>>

The gate electrode forming step is appropriately selected depending onthe intended purpose without any limitation, provided that it containsforming a gate electrode on the gate insulating layer. Examples thereofinclude methods listed as the gate electrode forming step in the firstproduction method.

In the second production method, an order for carrying out the sourceelectrode and drain electrode forming step, and the active layer formingstep is not restricted. The active layer forming step may be performedafter the source electrode and drain electrode forming step.Alternatively, the source electrode and drain electrode forming step maybe performed after the active layer forming step.

When the active layer forming step is performed after the sourceelectrode and drain electrode forming step in the second productionmethod, a top gate/bottom contact field-effect transistor can beproduced.

When the source electrode and drain electrode forming step is performedafter the active layer forming step in the second production method, atop gate/top contact field-effect transistor can be produced.

In accordance with the field-effect transistor of the present invention,the electron carrier density of the n-type oxide semiconductor is high(4.0×10¹⁷ cm⁻³ or greater), and the work function of the source anddrain electrodes in contact with the n-type oxide semiconductor is 4.90eV or greater. Therefore, the field-effect transistor of the presentinvention can achieve high field-effect mobility, a high on/off ratio,and a small absolute value of turn-on voltage.

Since the work function of the source and drain electrodes is 4.90 eV orgreater in the field-effect transistor of the present invention, theelectrodes are not oxidized by a heat treatment or oxidizing atmospheretreatment performed after the formation of the field-effect transistor,and the field-effect transistor the properties of which do notdeteriorate can be attained. Moreover, use of a metal having lowspecific resistance as an electrode material can keep the resistance ofthe lines low, and thus does not cause a problem of signal delay.

Moreover, it is not necessary to provide a step for introducing a bufferlayer at a contact interface between a n-type oxide semiconductor andsource and drain electrode for improving electric contact between then-type oxide semiconductor and the source and drain electrodes, and thesource and drain electrodes and lines can be formed with the identicalmaterial. Therefore, the production process is simple.

Moreover, the source and drain electrodes having the work function of4.90 eV or greater can be formed as a thin film having a low specificresistance by applying a coating liquid containing metal particles,alloy particles, an organometallic compound, or any combination thereof,followed by baking. Therefore, a printing process, such as a dropletejecting system (e.g., an inkjet system) can be applied for a productionprocess of the field-effect transistor, and thus a production method,which is simple and can form a film of a large area, can be selected.

(Display Element)

The display element of the present invention contains at least a lightcontrol element, and a driving circuit configured to drive the lightcontrol element, may further contain other members, according to thenecessity.

<Light Control Element>

The light control element is appropriately selected depending on theintended purpose without any limitation, provided that it is a elementconfigured to control light output according to a driving signal.Examples thereof include an electroluminescent (EL) element, anelectrochromic (EC) element, a liquid crystal element, anelectrophoretic element, and an electrowetting element.

<Driving Circuit>

The driving circuit is appropriately selected depending on the intendedpurpose without any limitation, provided that it contains thefield-effect transistor of the present invention.

<Other Members>

Other members are appropriately selected depending on the intendedpurpose without any limitation.

Since the display element contains the field-effect transistor of thepresent invention, high speed drive is realized, service life thereof islong, and variations between the elements can be reduced. Even when achange in the display element occurs with time, a driving transistor canbe operated at constant gate electrode.

(Image Display Device)

The image display device of the present invention contains at least aplurality of display elements, a plurality of lines, and a displaycontrol device, and may further contain other members, according to thenecessity.

<Plurality of Display Elements>

The plurality of the display elements are appropriately selecteddepending on the intended purpose without any limitation, provided thatthey are a plurality of the display elements of the present inventionthat are arranged in a matrix form.

<Plurality of Lines>

A plurality of the lines are appropriately selected depending on theintended purpose without any limitation, provided that they are capableof individually applying gate voltage and image data signal to eachfield-effect transistor in the display elements.

<Display Control Device>

The display control device is appropriately selected depending on theintended purpose without any limitation, provided that it canindividually control gate voltage and signal voltage of eachfield-effect transistor according to image data through the lines.

<Other Members>

Other members are appropriately selected depending on the intendedpurpose without any limitation.

Since the image display device contains the display element of thepresent invention, variations between the elements can be reduced, and alarge-screen image of high quality can be displayed.

(System)

The system of the present invention contains at least the image displaydevice of the present invention, and an image data generating device.

The image data generating device is configured to generate an image databased on image information to be displayed, and output the generatedimage data to the image display device.

Since the system is equipped with the image display device of thepresent invention, image information can be highly precisely displayed.

The display element, image display device, and system of the presentinvention are explained with reference to the drawings hereinafter.

First, a television device is explained as the system of the presentinvention with reference to FIG. 10.

In FIG. 10, a television device 100 is equipped with a main controldevice 101, a tuner 103, an AD converter (ADC) 104, a demodulatingcircuit 105, a TS (Transport Stream) decoder 106, a sound decoder 111, aDA converter (DAC) 112, a sound output circuit 113, a speaker 114, animage decoder 121, an image-OSD synthesis circuit 122, an image outputcircuit 123, an image display device 124, an OSD drawing circuit 125, amemory 131, an operating device 132, a drive interface (a drive IF) 141,a hard disk device 142, an optical disk device 143, an IR photodetector151, and a communication control unit 152.

The image decoder 121, the image-OSD synthesis circuit 122, the imageoutput circuit 123, and the OSD drawing circuit 125 constitute the imagedata generating device.

The main control device 101 is composed of CPU, flash ROM, and RAM, andis configured to control the entire television device 100.

In the flash ROM, a program written with a code that can be decoded withthe CPU, and various data used for processing in the CPU are stored.

Moreover, RAM is a memory for operations.

The tuner 103 is configured to select channels, which have been set inadvance, from the broadcast wave received by an aerial 210.

The ADC 104 is configured to convert the output signal (analoginformation) of the tuner 103 into digital information.

The demodulating circuit 105 is configured to demodulate the digitalinformation from the ADC 104.

The TS decoder 106 is configured to TS decode the output signal of thedemodulating circuit 105 to separate into sound information and imageinformation.

The sound decoder 111 is configured to decode the sound information fromthe TS decoder 106.

The DA converter (DAC) 112 is configured to convert the output signal ofthe sound decoder 111 into analog signal.

The sound output circuit 113 is configured to output the output signalof the DA converter (DAC) 112 to the speaker 114.

The image decoder 121 is configured to decode the image information fromthe TS decoder 106.

The image-OSD synthesis circuit 122 is configured to synthesize anoutput signal of the image decoder 121 and an output signal of the OSDdrawing circuit 125.

The image output circuit 123 is configured to output the output signalsof the image-OSD synthesis circuit 122 to the image display device 124.

The OSD drawing circuit 125 is equipped with a character generator todisplay characters or graphics on a screen of the image display device124, and is configured to generate a signal including displayinformation based on the instructions from the operating device 132 andthe IR photodetector 151.

The memory 131 is configured to temporarily store audio-visual (AV)data.

The operating device 132 is equipped with an input medium (notillustrated), such as a control panel, and is configured to informvarious information, which has been input by a user, to the main controldevice 101.

The drive IF 141 is an interactive communication interface. As oneexample, the drive IF is according to AT attachment packet interface(ATAPI).

The hard disk device 142 is composed of a hard disk, and a drivingdevice configured to drive the hard disk. The driving device recordsdata on the hard disk, as well as reproducing the data recorded in thehard disk.

The optical disk device 143 records data on an optical disk (e.g., DVD),as well as reproducing the data recorded on the optical disk.

The IR photodetector 151 receives photosignal from a remote-controlledtransmitter 220, and reports to the main control device 101.

The communication control unit 152 controls communication with internet.Various types of information can be obtained via internet.

FIG. 11 is a schematic diagram illustrating one example of the imagedisplay device of the present invention.

FIG. 11, the image display device 124 contains a display unit 300, and adisplay control device 400.

As illustrated in FIG. 12, the display unit 300 contains a display 310,in which a plurality (the number “n”×the number “m” in this case) of thedisplay elements 302 are arranged in a matrix.

As illustrated in FIG. 13, moreover, the display 310 contains “n” numberof scanning lines (X0, X1, X2, X3, . . . Xn−2, Xn−1) arranged along theX axis direction with a constant interval, “m” number of data lines (Y0,Y1, Y2, Y3, . . . Ym−1) arranged along the Y axis direction with aconstant interval, and “m” number of current supply lines (Y0 i, Y1 i,Y2 i, Y3 i, . . . Ym−1i) arranged along the Y axis direction with aconstant interval.

As described above, the display element can be specified with thescanning line and the data line.

The display element of the present invention is explained with referenceto FIG. 14, hereinafter.

FIG. 14 is a schematic diagram illustrating one example of the displayelement of the present invention.

As illustrated as one example in FIG. 14, the display element containsan organic electroluminescent (EL) element 350, and a driving circuit320 configured to emit light from the organic EL element 350.Specifically, the display 310 is an organic EL display of a so-calledactive matrix system. Moreover, the display 310 is a 32-inch colordisplay. Note that, a size of the display is not limited to theaforementioned size.

FIG. 15 illustrates one example of a positional relationship between anorganic EL element 350 and a field-effect transistor 20 as a drivingcircuit in a display element 302. In this example, the organic Elelement 350 is provided next to the field-effect transistor 20. Notethat, the field-effect transistor 10 and a capacitor (not illustrated)are formed on the identical substrate.

Although it is not illustrated in FIG. 15, it is preferred that apassivation film is provided above the active layer 22. As for amaterial of the passivation film, SiO₂, SiN_(x), Al₂O₃, or afluoropolymer is suitably used.

As illustrated in FIG. 16, for example, the organic EL element 350 maybe provided on the field-effect transistor 20. In this case,transparency is required for the gate electrode 26. As for the gateelectrode 26, therefore, a transparent electroconductive oxide, such asITO, In₂O₃, SnO₂, ZnO, Ga-doped ZnO, Al-doped ZnO, and Sb-doped SnO₂, isused. Note that, the reference number 360 represents an interlayerinsulating film (a leveling film). As for the interlayer insulatingfilm, polyimide, or an acrylic resin can be used.

FIG. 17 is a schematic diagram illustrating one example of an organic ELelement.

In FIG. 17, the organic EL element 350 contains a cathode 312, an anode314, and an organic EL thin film layer 340.

A material of the cathode 312 is appropriately selected depending on theintended purpose without any limitation, and examples thereof includealuminum (Al), magnesium (Mg)-silver (Ag) alloy, aluminum (Al)-lithium(Li) alloy, and indium tin oxide (ITO). Note that, the magnesium(Mg)-silver (Ag) alloy forms a high reflectance electrode with asufficient thickness thereof, and an extremely thin film (less thanabout 20 nm) thereof forms a semi-transparent electrode. In FIG. 17,light is taken out from the side of the anode, but light can be takenout from the side of the cathode, by making the cathode a transparent orsemi-transparent electrode.

A material of the anode 314 is appropriately selected depending on theintended purpose without any limitation, and examples thereof includeindium tin oxide (ITO), indium zinc oxide (IZO), and silver(Ag)-neodymium (Nd) alloy. Note that, in the case where the silver alloyis used, a resulting electrode becomes a high reflectance electrode,which is suitable for taking light out from the side of the cathode.

The organic EL thin film layer 340 contains an electron transportinglayer 342, a light emitting layer 344, and a hole transporting layer346. The electron transporting layer 342 is connected to the cathode312, and the hole transporting layer 346 is connected to the anode 314.The light emitting layer 344 emits light, as the predetermined voltageis applied between the anode 314 and the cathode 312.

Here, the electron transporting layer 342 and the light emitting layer344 may form one layer. Moreover, an electron injecting layer may beprovided between the electron transporting layer 342 and the cathode312. Further, a hole injecting layer may be provided between the holetransporting layer 346 and the anode 314.

As for the light control element, moreover, the so-called “bottomemission” organic EL element, in which light is taken out from the sideof the substrate, is explained above. However, the light control elementmay be a “top emission” organic EL element, in which light is taken outfrom the opposite side to the substrate.

The driving circuit 320 in FIG. 14 is explained.

The driving circuit 320 contains two field-effect transistors 10, 20,and a capacitor 30.

The field-effect transistor 10 functions as a switching element. Thegate electrode G of the field-effect transistor 10 is connected to thepredetermined scanning line, and the source electrode S of thefield-effect transistor 10 is connected to the predetermined data line.Moreover, the drain electrode D of the field-effect transistor 10 isconnected to one terminal of the capacitor 30.

The field-effect transistor 20 is configured to supply electric currentto the organic EL element 350. The gate electrode G of the field-effecttransistor 20 is connected to the drain electrode D of the field-effecttransistor 10. The drain electrode D of the field-effect transistor 20is connected to the anode 314 of the organic EL element 350, and thesource electrode S of the field-effect transistor 20 is connected to thepredetermined current supply line.

The capacitor 30 is configured to store a state of the field-effecttransistor 10, i.e., data. The other terminal of the capacitor 30 isconnected to the predetermined current supply line.

As the field-effect transistor 10 is turned in the state of “On,” theimage data is stored in the capacitor 30 via the signal line Y2. Evenafter turning the field-effect transistor 10 in the state of “Off,” thefield-effect transistor 20 is maintained in the state of “On”corresponding to the image data so that the organic EL element 350 isdriven.

FIG. 18 is a schematic diagram illustrating another example of the imagedisplay device of the present invention.

In FIG. 18, the image display device contains a display element 302,lines (scanning lines, data lines, and current supply lines), and adisplay control device 400.

The display control device 400 contains an image data processing circuit402, a scanning line driving circuit 404, and a data line drivingcircuit 406.

The image data processing circuit 402 judges luminance of a plurality ofthe display elements 302 in the display based on output signal of theimage output circuit 123.

The scanning line driving circuit 404 individually applies voltage tothe number “n” of scanning lines according to the instructions of theimage data processing circuit 402.

The data line driving circuit 406 individually applies voltage to thenumber “m” of data lines according to the instruction of the image dataprocessing circuit 402.

The embodiment above explains the case where the light control elementis an organic EL element, but the light control element is not limitedto the organic EL element. For example, the light control element may bean electrochromic element. In this case, the display is anelectrochromic display.

Moreover, the light control element may be a liquid crystal element. Inthis case, the display is a liquid crystal display, and a current supplyline is not necessary to the display element 302′ as illustrated in FIG.19. As illustrated in FIG. 20, moreover, the driving circuit 320′ may becomposed of one field-effect transistor 40, which is identical to thefield-effect transistors 10 and 20. In the field-effect transistor 40,the gate electrode G is connected to the predetermined scanning line,and the source electrode S is connected to the predetermined data line.Moreover, the drain electrode D is connected to the capacitor 361 and apixel electrode of the liquid crystal element 370.

Moreover, the light control element may be an electrophoretic element,an inorganic EL element, or an electrowetting element.

The case where the system of the present invention is a televisiondevice is explained above, but the system is not limited as long as thesystem contains the image display device 124 as a device for displayingimages and information. For example, the system may be a computersystem, in which a computer (including a personal computer) is connectedto the image display device 124.

Moreover, the image display device 124 can be used as a display unit ina mobile information device (e.g., a mobile phone, a portable musicplayer, a portable video player, an electronic book, a personal digitalassistant (PDA)), or a camera device (e.g., a still camera, a videocamera). The image display device 124 can be used as a display unit forvarious types of information in a transport system (e.g., a car, an aircraft, a train, and a ship). Furthermore, the image display device 124can be used as a display unit for various types of information in ameasuring device, an analysis device, a medical equipment, oradvertising media.

EXAMPLES

Examples of the present invention are explained hereinafter, but thefollowing examples shall not be construed as to limit the scope of thepresent invention in any way.

Example 1 Production of Field-Effect Transistor

—Formation of Gate Electrode—

On a glass substrate, Al was deposited to have a thickness of 100 nm byvapor deposition, and then the deposited Al was patterned into the formof a line through photolithography and etching, to thereby form a gateelectrode.

—Formation of Gate Insulating Layer—

A film of SiO₂ was formed by plasma CVD to have a thickness of 200 nm,to thereby form a gate insulating layer.

—Formation of Active Layer—

On the formed insulating layer, a Mg—In based oxide semiconductor film(active layer) was formed by sputtering in the method described inExamples of JP-A No. 2010-74148. As for a target, a polycrystallinesintered compact having a composition of In₂MgO₄ was used. The backpressure in the sputtering chamber was set to 2×10⁻⁵ Pa. The totalpressure was set to 0.3 Pa by adjusting flow rates of argon gas andoxygen gas supplied during the sputtering. An amount of oxygen in theoxide semiconductor film and the electron carrier density werecontrolled by adjusting the flow rate of the oxygen gas. The oxygen flowrate during the sputtering is depicted in Table 1.

A thickness of the obtained oxide semiconductor film (active layer) was50 nm.

—Formation of Source Electrode and Drain Electrode—

On the formed active layer, a gold nanometal ink (Au-1Chb, manufacturedby ULVAC, Inc., the average particle diameter: 5 nm, the metal content:50% by mass) was applied into the predetermined pattern by means of aninkjet device. The applied ink was heated in the air for 30 minutes at300° C., to form a source electrode and a drain electrode each having athickness of 100 nm. A channel width was 400 μm, and a channel lengthspecified with a length between the source electrode and the drainelectrode was 50 μm.

In the manner as described above, a bottom gate/top contact field-effecttransistor was produced.

—Production of Element Used for Work Function Measurement—

In order to measure work function, an element used for work functionmeasurement was obtained by forming a metal film on a glass substrate inthe same manner as in the formation of the source electrode and drainelectrode.

<Production of Element for Hall Effect Measurement>

—Formation of n-Type Oxide Semiconductor Film—

In the same manner as in the aforementioned formation of the activelayer of the field-effect transistor, a square pattern of a n-type oxidesemiconductor having a side of 8 mm was formed on a glass substratethrough a shadow mask. The conditions for the sputtering were set thesame as in the formation of the active layer. The conditions forsputtering are depicted in Table 1.

—Formation of Contact Electrode—

A contact electrode for hall effect measurement was formed on the glasssubstrate, on which the n-type oxide semiconductor film had been formed,by vacuum deposition using a shadow mask. As for a deposition source, Alwas used.

<Evaluation of Electron Carrier Density (Concentration)>

The element for hall effect measurement was subjected to themeasurements of specific resistance and hall effect by means of a halleffect measurement system (ResiTest8300, manufactured by TOYOCorporation), to thereby determined an electron carrier density of(cm⁻³) the n-type oxide semiconductor. The obtained electron carrierdensity (concentration) is depicted in Table 1.

<Evaluation of Work Function>

The work function of the metal film of the element for work functionmeasurement was determined by means of a photoelectron spectrometer inair AC-2 (manufactured by RIKEN KEIKI Co., Ltd.). The obtained workfunction is depicted in Table 1.

<Evaluation of Transistor Performance>

An evaluation of transistor performance was performed on the obtainedfield-effect transistor by means of a semiconductor parameter analyzer(Semiconductor Parameter Analyzer 4156C, manufactured by AgilentTechnologies, Inc.). Current-voltage characteristics were evaluated bysetting the source-drain voltage Vds to 20 V, and varying the gatevoltage Vg from −30 V to +30 V. The field-effect mobility was calculatedin the saturated region. Moreover, a ratio (on/off ratio) of thesource-drain current Ids of the transistor in the “on” state (e.g.,Vg=20 V) to that in the “off” state (e.g., Vg=−20 V) was calculated.Moreover, the gate voltage (turn-on voltage), with which thesource-drain current Ids turned to increase was calculated. The resultsare depicted in Table 1.

Note that, FIG. 7 is a diagram depicting current-voltage characteristicsof Example 1 with the carrier density of 1.0×10¹⁸ cm⁻³. Moreover, FIG. 6is a diagram depicting current-voltage characteristics of ComparativeExample 1 with the carrier density of 5.9×10¹⁵ cm⁻³.

Example 2

A bottom gate/top contact field-effect transistor was produced in thesame manner as in Example 1, provided that the formation of the sourceelectrode and drain electrode was changed to the following method.Moreover, the evaluations were carried out in the same manner as inExample 1. The results are depicted in Table 2.

—Formation of Source Electrode and Drain Electrode—

On the formed active layer, a silver nanometal ink (NPS-J, manufacturedby Harima Chemical Group, Inc., the average particle diameter: 5 nm, themetal content: 59% by mass) was applied into the predetermined patternby means of an inkjet device. The applied ink was heated in the air for30 minutes at 220° C., to thereby form a source electrode and a drainelectrode each having a thickness of 100 nm. A channel width was 400 μm,and a channel length specified with a length between the sourceelectrode and the drain electrode was 50 μm.

Example 3

A bottom gate/top contact field-effect transistor was produced in thesame manner as in Example 1, provided that the formation of the sourceelectrode and drain electrode was changed to the following method.Moreover, the evaluations were carried out in the same manner as inExample 1. The results are depicted in Table 3.

—Formation of Source Electrode and Drain Electrode—

On the formed active layer, a silver-palladium nanoparticle paste(NAGNPD15-K04, manufactured by DAIKEN CHEMICAL CO., LTD., the averageparticle diameter: 4 nm to 10 nm, the metal content: 21% by mass) wasapplied into the predetermined pattern by means of an inkjet device. Theapplied ink was heated in the art for 30 minutes at 300° C., to therebyform a source electrode and a drain electrode each having a thickness of100 nm. A channel width was 400 μm, and a channel length specified witha length between the source electrode and the drain electrode was 50 μm.

Example 4

A bottom gate/bottom contact field-effect transistor was produced in thesame manner as in Example 1, provided that the formation of the sourceelectrode and drain electrode and the formation of the active layer werechanged to the following methods. Moreover, the evaluations were carriedout in the same manner as in Example 1. The results are depicted inTable 4.

—Formation of Source Electrode and Drain Electrode—

On the formed gate insulating layer, a source electrode and a drainelectrode was formed using Pt by photolithography and a lift-offprocess. The Pt film was formed by sputtering. As for the sputtering,the sputtering powder was 200 W, the pressure at the time of forming thefilm was 0.35 Pa, and the thickness of the film was 50 nm. A channellength specified with a length between the source electrode and thedrain electrode was 50 μm.

—Formation of Active Layer—

On the formed source electrode and drain electrode as well as the gateinsulating layer, a Mg—In based oxide semiconductor film (active layer)was formed by sputtering in the method described in JP-A No. 2010-74148.As for a target, a polycrystalline sintered compact having a compositionof In₂MgO₄. The back pressure in the sputtering chamber was set to2×10⁻⁵ Pa. The total pressure was set to 0.3 Pa by adjusting flow ratesof argon gas and oxygen gas supplied during the sputtering. An amount ofoxygen in the oxide semiconductor film and the electron carrier densitywere controlled by adjusting the flow rate of the oxygen gas. The oxygenflow rate during the sputtering is depicted in Table 4.

A thickness of the obtained oxide semiconductor film (active layer) was50 nm.

Example 5

A bottom gate/top contact field-effect transistor was produced in thesame manner as in Example 1, provided that the formation of the sourceelectrode and drain electrode was changed to the following method.Moreover, the evaluations were carried out in the same manner as inExample 1. The results are depicted in Table 5.

—Formation of Source Electrode and Drain Electrode—

On the formed active layer, an Au resinate solution (an organometalliccompound, manufactured by DAIKEN CHEMICAL CO., LTD., metal content: 20%by mass) was applied into the predetermined pattern by means of aninkjet device. The applied ink was heated in the art for 60 minutes at300° C., to thereby form a source electrode a drain electrode eachhaving a thickness of 100 nm. A channel width was 400 μm, and a channellength specified with a length between the source electrode and thedrain electrode was 50 μm.

Example 6

A bottom gate/top contact field-effect transistor was produced in thesame manner as in Example 1, provided that the formation of the sourceelectrode and drain electrode was changed to the following method.Moreover, the evaluations were carried out in the same manner as inExample 1. The results are depicted in Table 6.

—Formation of Source Electrode and Drain Electrode—

On the formed active layer, a Pt resinate solution (an organic platinumcompound, manufactured by DAIKEN CHEMICAL CO., LTD., metal content: 10%by mass) was applied into the predetermined pattern by means of aninkjet device. The applied ink was heated in the air for 60 minutes at300° C., to thereby form a source electrode and a drain electrode eachhaving a thickness of 100 nm. A channel width was 400 μm, and a channellength specified with a length between the source electrode and thedrain electrode was 50 μm.

Comparative Example 1

A bottom gate/bottom contact field-effect transistor was produced in thesame manner as in Example 1, provided that the formation of the sourceelectrode and drain electrode and the formation of the active layer werechanged to the following methods. Moreover, the evaluations were carriedout in the same manner as in Example 1. The results are depicted inTable 7.

—Formation of Source Electrode and Drain Electrode—

On the formed gate insulating layer, a source electrode and a drainelectrode each having a thickness of 100 nm were formed by vacuumdeposition. As for deposition source, gold was used. A channel width was400 μm, and a channel length was 50 μm.

—Formation of Active Layer—

On the formed source electrode and drain electrode as well as the gateinsulating layer, a Mg—In based oxide semiconductor film (active layer)was formed by sputtering in the method described in JP-A No. 2010-74148.As for a target, a polycrystalline sintered compact having a compositionof In₂MgO₄ was used. The back pressure in the sputtering chamber was setto 2×10⁻⁵ Pa. The total pressure was set to 0.3 Pa by adjusting flowrates of argon gas and oxygen gas supplied during the sputtering. Anamount of oxygen in the oxide semiconductor film and the electroncarrier density were controlled by adjusting the flow rate of the oxygengas. The oxygen flow rate during the sputtering is depicted in Table 7.

A thickness of the obtained oxide semiconductor film (active layer) was50 nm.

TABLE 1 Film coating condition, Carrier oxygen Carrier Source- Film Workmobility On/off Turn-on flow rate density drain forming function μFEratio voltage (vol %) n (cm⁻³) electrode method (eV) (cm²/Vs) (×10⁸) (V)Ex. 1 10 4.7E+17 Au Coating 4.91 5.4 2.3 2.0 2.00 1.0E+18 5.6 4.7 1.01.00 1.2E+18 5.9 3.6 1.0 0.90 4.0E+18 6 5.7 0.5 0.80 5.1E+18 6.3 5.1 0.50.70 7.5E+18 8.4 2.8 0.5 0.60 9.5E+18 8.6 1.9 0.0 0.50 1.1E+19 8.8 1.30.0 0.35 2.1E+19 8.9 1.2 0.0 0.20 4.0E+19 9 1.1 0.0

Note that, “E” in Tables 1 to 16, 18 and 19 denotes “the exponent of10.” For example, “1.0E+17” is “1.0×10¹⁷,” and “1.0E-05” is “0.00001.” Edenotes the same in FIGS. 6 to 9, and 21 to 24.

TABLE 2 Film coating condition, Carrier oxygen Carrier Source- Film Workmobility On/off Turn-on flow rate density drain forming function μFEratio voltage (vol %) n (cm⁻³) electrode method (eV) (cm²/Vs) (×10⁸) (V)Ex. 2 10 4.7E+17 Ag Coating 5.10 4.2 5.2 3.0 2.00 1.0E+18 4.3 2.8 3.01.00 1.2E+18 4.5 3.7 2.0 0.90 4.0E+18 5.1 2.6 2.0 0.80 5.1E+18 5.6 4.82.0 0.70 7.5E+18 8.6 3.1 2.0 0.60 9.5E+18 9.2 4.1 1.0 0.50 1.1E+19 10.13.4 1.0 0.35 2.1E+19 10.7 2.8 1.0 0.20 4.0E+19 10.7 2.8 0.0

TABLE 3 Film coating condition, Carrier oxygen Carrier Source- Film Workmobility On/off Turn-on flow rate density drain forming function μFEratio voltage (vol %) n (cm⁻³) electrode method (eV) (cm²/Vs) (×10⁸) (V)Ex. 3 10 4.7E+17 Ag/Pd Coating 5.20 2.5 6.6 3.0 2.00 1.0E+18 3.2 4.3 3.01.00 1.2E+18 3.5 5.6 3.0 0.90 4.0E+18 3.9 4.7 2.0 0.80 5.1E+18 6.1 3.82.0 0.70 7.5E+18 7.5 1.4 2.0 0.60 9.5E+18 9.4 3.3 2.0 0.50 1.1E+19 10.61.8 1.0 0.35 2.1E+19 10.9 1.2 1.0 0.20 4.0E+19 10.9 1.2 0.0

TABLE 4 Film coating condition, Carrier oxygen Carrier Source- Film Workmobility On/off Turn-on flow rate density drain forming function μFEratio voltage (vol %) n (cm⁻³) electrode method (eV) (cm²/Vs) (×10⁸) (V)Ex. 4 10 4.7E+17 Pt Vacuum 5.30 2.4 8.9 3.0 2.00 1.0E+18 deposition 3.28.7 3.0 1.00 1.2E+18 3.4 7.4 2.5 0.90 4.0E+18 3.8 7.1 2.5 0.80 5.1E+186.3 6.4 2.5 0.70 7.5E+18 7.6 5.7 2.5 0.60 9.5E+18 9.4 6.1 2.0 0.501.1E+19 10.5 5.5 2.0 0.35 2.1E+19 11.1 3.9 1.0 0.20 4.0E+19 11.1 3.9 1.0

TABLE 5 Film coating condition, Carrier oxygen Carrier Source- Film Workmobility On/off Turn-on flow rate density drain forming function μFEratio voltage (vol %) n (cm⁻³) electrode method (eV) (cm²/Vs) (×10⁸) (V)Ex. 5 10 4.7E+17 Au Coating 4.95 4.3 4.1 2.0 2.00 1.0E+18 4.3 3.2 1.01.00 1.2E+18 4.6 2.6 1.0 0.90 4.0E+18 4.8 2.4 1.0 0.80 5.1E+18 5.2 1.51.0 0.70 7.5E+18 6.3 6.2 0.5 0.60 9.5E+18 7.7 1.1 0.5 0.50 1.1E+19 7.91.2 0.0 0.35 2.1E+19 8.2 1.1 0.0 0.20 4.0E+19 10.3 1.1 0.0

TABLE 6 Film coating condition, Carrier oxygen Carrier Source- Film Workmobility On/off Turn-on flow rate density drain forming function μFEratio voltage (vol %) n (cm⁻³) electrode method (eV) (cm²/Vs) (×10⁸) (V)Ex. 6 10 4.7E+17 Pt Coating 5.40 3.6 3.6 3.0 2.00 1.0E+18 4.1 2.9 3.01.00 1.2E+18 4.1 4.8 2.5 0.90 4.0E+18 4.6 4.7 2.5 0.80 5.1E+18 5.8 3.52.0 0.70 7.5E+18 6.4 6.1 2.0 0.60 9.5E+18 7.8 7.4 2.0 0.50 1.1E+19 9.31.8 1.5 0.35 2.1E+19 9.8 2.4 0.5 0.20 4.0E+19 10.4 2.7 0.0

TABLE 7 Film coating condition, Carrier oxygen Carrier Source- Film Workmobility On/off Turn-on flow rate density drain forming function μFEratio voltage (vol %) n (cm⁻³) electrode method (eV) (cm²/Vs) (×10⁸) (V)Comp. 50 5.9E+15 Au Vacuum 4.74 — — — Ex. 1 10 4.7E+17 deposition 1.80.3 −0.5 2.00 1.0E+18 1.9 1.2 −1.0 1.00 1.2E+18 2.1 1.4 −2.0 0.904.0E+18 2.5 2.3 −4.0 0.80 5.1E+18 5.3 1.2 −5.0 0.70 7.5E+18 6.1 0.2 −8.00.60 9.5E+18 7.5 0.025 −15 0.50 1.1E+19 — 0.0001 — 0.35 2.1E+19 —0.00001 — 0.20 4.0E+19 — 0.000001 —

As for TFT used as a switching element, the field-effect mobility(carrier mobility) thereof is preferably 1.0 cm²/Vs or greater, morepreferably 2.0 cm²/Vs or greater. Moreover, the on/off ratio ispreferably 1×10⁶ or greater, more preferably 1×10⁷ or greater, andparticularly preferably 1.0×10⁸ or greater. The absolute value of theturn-on voltage is preferably 5 V or lower, more preferably 3 V orlower. Moreover, the normally-off properties with which the turn-onvoltage is 0 V or greater is particularly preferable.

Tables 1 to 7 depict the film forming conditions of the oxidesemiconductor films formed in Examples 1 to 6 and Comparative Example 1,the carrier density of the oxide semiconductor films, types of sourceand drain electrodes, the forming methods of the source and drainelectrode, the work functions of the metal films formed in the samemanner as the source and drain electrodes, and the transistorcharacteristics of the field-effect transistors.

Examples 1 to 6 show the results when Au, Ag, Ag/Pd, or Pt formed bycoating, or Pt formed by vacuum deposition is used as source and drainelectrodes. It could be understood from the results of the transistorcharacteristics depicted in Tables 1 to 6 that a high field-effectmobility, a high on/off ratio, and a small absolute value of the turn-onvoltage were attained with the electron carrier density in the range of4.0×10¹⁷ cm⁻³ to 4.0×10¹⁹ cm⁻³. Since the electron carrier density ofthe oxide semiconductor was high, contact resistance between the metaland the oxide semiconductor was not large at the contact interface, andtherefore the desirable device properties were exhibited.

The transistor using Au formed by coating had a high on/off ratio, andnormally-off transistor properties where a small absolute value of theturn-on voltage even with the oxide semiconductor having the electroncarrier density of 4.0×10¹⁷ cm⁻³ or greater. This is because the workfunction of Au formed by coating is higher than the work function of Auformed by vacuum deposition.

On the other hand, the results of Comparative Example 1 with theelectron carrier density (concentration) of 5.9×10¹⁵ cm⁻³ shows theresults of a transistor using a n-type oxide semiconductor having theelectron carrier density (concentration) of 1×10¹⁵ cm⁻³ to 1×10¹⁶ cm⁻³,which is typically suitably used for a transistor using a n-type oxidesemiconductor, and gold formed by vacuum deposition is used as sourceand drain electrodes. In this case, the on-current is low, i.e. about1×10⁹ A, and the transistor hardly functions. This is because Schottkybarrier junction is formed at a plane where the gold having a relativelyhigh work function and the n-type oxide semiconductor are in contact,and thus contact resistance becomes large.

Other than above, Comparative Example 1 shows the results of thetransistor using the n-type oxide semiconductor with electron carrierdensity of 4.0×10¹⁷ cm⁻³ or greater, and gold formed by vacuumdeposition is used as source and drain electrode. In the case where theelectron carrier density is in the range of 4.0×10¹⁷ cm⁻³ or greater butless than 1.2×10¹⁸ cm⁻³, transistor properties that the field-effectmobility is high, an on/off ratio is high, and an absolute value of theturn-on voltage is small are obtained. However, the turn-on voltage isnegative, that means it is not normally-off operation. When the electroncarrier density is equal to or greater than the aforementioned range(4.0×10¹⁸ cm⁻³ or greater), the absolute value of the turn-on voltagetends to be large. When the electron carrier density is 1.1×10¹⁹ cm⁻³ orgreater, the transistor operation cannot be attained.

Example 7

A bottom gate/top contact field-effect transistor was produced in thesame manner as in Example 1, provided that the formation of the activelayer was changed to the following method. Moreover, the evaluationswere carried out in the same manner as in Example 1. The results aredepicted in Table 8.

—Formation of Active Layer—

On the formed insulating layer, an In—Ga—Zn based oxide thin film wasformed by DC sputtering.

As for a target, a polycrystalline sintered compact of In—Ga—Zn—O togive a composition ratio of In:Ga:Zn=1:1:1 was used. The sputteringpower was set to 140 W, the pressure during the formation of the filmwas set to 0.69 Pa, and the temperature of the substrate was notcontrolled. The total pressure was set to 0.69 Pa by adjusting flowrates of argon gas and oxygen gas supplied during the sputtering. Theduration for forming the film was 20 minutes, and the thickness of thefilm was 70 nm. An amount of oxygen in the oxide semiconductor filmcould be controlled by adjusting the oxygen flow rate, to therebycontrol the electron carrier density. The oxygen flow rate during thesputtering is depicted in Table 8.

Example 8

A bottom gate/top contact field-effect transistor was produced in thesame manner as in Example 7, provided that the formation of the sourceelectrode and drain electrode was changed to the following method.Moreover, the evaluations were carried out in the same manner as inExample 1. The results are depicted in Table 9.

—Formation of Source Electrode and Drain Electrode—

On the formed active layer, a silver nanometal ink (NPS-J, manufacturedby Harima Chemical Group, Inc., the average particle diameter: 5 nm, themetal content: 59% by mass) was applied into the predetermined patternby means of an inkjet device. The applied ink was heated in the air for30 minutes at 220° C., to thereby form a source electrode and a drainelectrode each having a thickness of 100 nm. A channel width was 400 μm,and a channel length specified with a length between the sourceelectrode and the drain electrode was 50 μm.

Example 9

A bottom gate/top contact field-effect transistor was produced in thesame manner as in Example 7, provided that the formation of the sourceelectrode and drain electrode was changed to the following method.Moreover, the evaluations were carried out in the same manner as inExample 1. The results are depicted in Table 10.

—Formation of Source Electrode and Drain Electrode—

On the formed active layer, a silver-palladium nanoparticle paste(NAGNPD15-K04, manufactured by DAIKEN CHEMICAL CO., LTD., the averageparticle diameter: 4 nm to 10 nm, the metal content: 21% by mass) wasapplied into the predetermined pattern by means of an inkjet device. Theapplied ink was heated in the air for 30 minutes at 300° C., to therebyform a source electrode and a drain electrode each having a thickness of100 nm. A channel width was 400 μm, and a channel length specified witha length between the source electrode and the drain electrode was 50 μm.

Example 10

A bottom gate/bottom contact field-effect transistor was produced in thesame manner as in Example 7, provided that the formation of the sourceelectrode and drain electrode and the formation of the active layer werechanged to the following methods. Moreover, the evaluations were carriedout in the same manner as in Example 1. The results are depicted inTable 11.

—Formation of Source Electrode and Drain Electrode—

On the formed gate insulating layer, a source electrode and a drainelectrode was formed using Pt by photolithography and a lift-offprocess. The Pt film was formed by sputtering. As for the sputtering,the sputtering powder was 200 W, the pressure at the time of forming thefilm was 0.35 Pa, and the thickness of the film was 50 nm. A channellength specified with a length between the source electrode and thedrain electrode was 50 μm.

—Formation of Active Layer—

On the formed source electrode and drain electrode as well as the gateinsulating layer, an In—Ga—Zn based oxide thin film was formed by DCsputtering.

As for a target, a sintered compact of to give a composition ratio ofIn:Ga:Zn=1:1:1 was used. The sputtering power was set to 140 W, thepressure during the formation of the film was set to 0.69 Pa, and thetemperature of the substrate was not controlled. The total pressure wasset to 0.69 Pa by adjusting flow rates of argon gas and oxygen gas fedduring the sputtering. The duration for forming the film was 20 minutes,and the thickness of the film was 70 nm. An amount of oxygen in theoxide semiconductor film could be controlled by adjusting the oxygenflow rate, to thereby control the electron carrier density. The oxygenflow rate during the sputtering is depicted in Table 11.

Example 11

A bottom gate/top contact field-effect transistor was produced in thesame manner as in Example 7, provided that the formation of the sourceelectrode and drain electrode was changed to the following method.Moreover, the evaluations were carried out in the same manner as inExample 1. The results are depicted in Table 12.

—Formation of Source Electrode and Drain Electrode—

On the formed active layer, an Au resinate solution (an organometalliccompound, manufactured by DAIKEN CHEMICAL CO., LTD., metal content: 20%by mass) was applied into the predetermined pattern by means of aninkjet device. The applied ink was heated in the air for 60 minutes at300° C., to thereby form a source electrode and a drain electrode eachhaving a thickness of 100 nm. A channel width was 400 μm, and a channellength specified with a length between the source electrode and thedrain electrode was 50 μm.

Example 12

A bottom gate/top contact field-effect transistor was produced in thesame manner as in Example 7, provided that the formation of the sourceelectrode and drain electrode was changed to the following method.Moreover, the evaluations were carried out in the same manner as inExample 1. The results are depicted in Table 13.

—Formation of Source Electrode and Drain Electrode—

On the formed active layer, a Pt resinate solution (an organic platinumcompound, manufactured by DAIKEN CHEMICAL CO., LTD., metal content: 10%by mass) was applied into the predetermined pattern by means of aninkjet device. The applied ink was heated in the air for 60 minutes at300° C., to thereby form a source electrode and a drain electrode eachhaving a thickness of 100 nm. A channel width was 400 μm, and a channellength specified with a length between the source electrode and thedrain electrode was 50 μm.

Comparative Example 2

A field-effect transistor was produced in the same manner as in Example7, provided that the formation of the source electrode and drainelectrode and the formation of the active layer were changed to thefollowing methods. Moreover, the evaluations were carried out in thesame manner as in Example 1. The results are depicted in Table 14.

—Formation of Source Electrode and Drain Electrode—

On the formed gate insulating layer, a source electrode and a drainelectrode each having a thickness of 100 nm were formed by vacuumdeposition. As for deposition source, gold was used. A channel width was400 μm, and a channel length was 50 μm.

—Formation of Active Layer—

On the formed source electrode and drain electrode as well as the gateinsulating layer, an In—Ga—Zn based oxide thin film was formed by DCsputtering.

As for a target, a sintered compact of In—Ga—Zn—O to give a compositionratio of In:Ga:Zn=1:1:1 was used. The sputtering power was set to 140 W,the pressure during the formation of the film was set to 0.69 Pa, andthe temperature of the substrate was not controlled. The total pressurewas set to 0.69 Pa by adjusting flow rates of argon gas and oxygen gasfed during the sputtering. The duration for forming the film was 20minutes, and the thickness of the film was 70 nm. An amount of oxygen inthe oxide semiconductor film could be controlled by adjusting the oxygenflow rate, to thereby control the electron carrier density. The oxygenflow rate during the sputtering is depicted in Table 14.

TABLE 8 Film coating condition, Carrier oxygen Carrier Source- Film Workmobility On/off Turn-on flow rate density drain forming function μFEratio voltage (vol %) n (cm⁻³) electrode method (eV) (cm²/Vs) (×10⁸) (V)Ex. 7 1.7 7.5E+17 Au Coating 4.91 7.7 3.9 1.0 1.5 1.2E+18 7.7 7.2 1.01.4 2.0E+18 7.8 5.6 0.5 1.3 5.2E+18 8.2 4.5 0.5 1.1 9.3E+18 8.5 4.3 0.51.0 1.5E+19 9.0 1.8 0.0 0.9 4.1E+19 11.2 3.6 0.0

TABLE 9 Film coating condition, Carrier oxygen Carrier Source- Film Workmobility On/off Turn-on flow rate density drain forming function μFEratio voltage (vol %) n (cm⁻³) electrode method (eV) (cm²/Vs) (×10⁸) (V)Ex. 8 1.7 7.5E+17 Ag Coating 5.10 7.6 2.8 1.0 1.5 1.2E+18 7.7 4.5 1.01.4 2.0E+18 8.1 4.1 1.0 1.3 5.2E+18 8.1 1.8 0.0 1.1 9.3E+18 8.7 2.9 0.01.0 1.5E+19 9.2 3.8 0.0 0.9 4.1E+19 10.3 2.4 0.0

TABLE 10 Film coating condition, Carrier oxygen Carrier Source- FilmWork mobility On/off Turn-on flow rate density drain forming functionμFE ratio voltage (vol %) n (cm⁻³) electrode method (eV) (cm²/Vs) (×10⁸)(V) Ex. 9 1.7 7.5E+17 Ag/Pd Coating 5.20 7.1 2.3 3.0 1.5 1.2E+18 7.2 3.73.0 1.4 2.0E+18 7.4 2.4 2.0 1.3 5.2E+18 7.6 1.9 2.0 1.1 9.3E+18 7.9 2.51.0 1.0 1.5E+19 9.1 3.8 1.0 0.9 4.1E+19 9.8 6.4 1.0

TABLE 11 Film coating condition, Carrier oxygen Carrier Source- FilmWork mobility On/off Turn-on flow rate density drain forming functionμFE ratio voltage (vol %) n (cm⁻³) electrode method (eV) (cm²/Vs) (×10⁸)(V) Ex. 10 1.7 7.5E+17 Pt Vacuum 5.30 6.9 5.6 3.0 1.5 1.2E+18 deposition7 6.4 3.0 1.4 2.0E+18 7 5.3 3.0 1.3 5.2E+18 7.5 7.8 2.5 1.1 9.3E+18 8.14.3 2.5 1.0 1.5E+19 8.3 6.5 2.0 0.9 4.1E+19 9.5 4.1 1.0

TABLE 12 Film coating condition, Carrier oxygen Carrier Source- FilmWork mobility On/off Turn-on flow rate density drain forming functionμFE ratio voltage (vol %) n (cm⁻³) electrode method (eV) (cm²/Vs) (×10⁸)(V) Ex. 11 1.7 7.5E+17 Au Coating 4.95 7.6 4.2 1.0 1.5 1.2E+18 7.6 3.91.0 1.4 2.0E+18 7.7 3.7 1.0 1.3 5.2E+18 8.1 3.1 1.0 1.1 9.3E+18 8.6 2.60.0 1.0 1.5E+19 8.9 2.1 0.0 0.9 4.1E+19 10.1 2.4 0.0

TABLE 13 Film coating condition, Carrier oxygen Carrier Source- FilmWork mobility On/off Turn-on flow rate density drain forming functionμFE ratio voltage (vol %) n (cm⁻³) electrode method (eV) (cm²/Vs) (×10⁸)(V) Ex. 12 1.7 7.5E+17 Pt Coating 5.40 6.1 4.7 3.0 1.5 1.2E+18 6.8 3.23.0 1.4 2.0E+18 6.9 5.1 2.5 1.3 5.2E+18 7.1 2.8 2.5 1.1 9.3E+18 7.5 3.62.5 1.0 1.5E+19 8.4 7.1 1.5 0.9 4.1E+19 8.7 5.6 0.5

TABLE 14 Film coating condition, Carrier oxygen Carrier Source- FilmWork mobility On/off Turn-on flow rate density drain forming functionμFE ratio voltage (vol %) n (cm⁻³) electrode method (eV) (cm²/Vs) (×10⁸)(V) Comp. 1.9 5.9E+16 Au Vacuum 4.77 0.2 0.03 3.0 Ex. 2 1.7 7.5E+17deposition 1.8 0.9 −0.5 1.5 1.2E+18 2.3 1.2 −1.0 1.4 2.0E+18 2.5 1.4−3.0 1.3 5.2E+18 3.4 0.7 −4.0 1.1 9.3E+18 5.8 0.1 −5.0 1.0 1.5E+19 6.20.04 −9.0 0.9 4.1E+19 — 0.00006 —

Tables 8 to 14 depict the film forming conditions of the oxidesemiconductor films formed in Examples 7 to 12 and Comparative Example2, the carrier density of the oxide semiconductor films, types of sourceand drain electrodes, the forming methods of the source and drainelectrode, the work functions of the metal films formed in the samemanner as the source and drain electrodes, and the transistorcharacteristics of the field-effect transistors.

Examples 7 to 12 show the results when Au, Ag, or Ag/Pd, formed bycoating, or Pt formed by vacuum deposition is used as source and drainelectrodes. It could be understood from the results of the transistorcharacteristics depicted in Tables 8 to 13 that a high field-effectmobility, a high on/off ratio, and a small absolute value of the turn-onvoltage were attained with the electron carrier density in the range of4.0×10¹⁷ cm⁻³ to 4.0×10¹⁹ cm⁻³. Since the electron carrier density ofthe oxide semiconductor was high, contact resistance between the metaland the oxide semiconductor was not large at the contact interface, andtherefore the desirable device properties were exhibited.

The transistor using Au formed by coating had a high on/off ratio, andnormally-off transistor properties where a small absolute value of theturn-on voltage even with the oxide semiconductor having the electroncarrier density of 4.0×10¹⁸ cm⁻³ or greater. This is because the workfunction of Au formed by coating is higher than the work function of Auformed by vacuum deposition.

On the other hand, the results of Comparative Example 2 with theelectron carrier density (concentration) of 5.9×10¹⁶ cm⁻³ show theresults of a transistor using a n-type oxide semiconductor, which hasthe carrier density close to the electron carrier density(concentration) of 1×10¹⁵ cm⁻³ to 1×10¹⁶ cm⁻³, which is typicallysuitably used, and gold formed by vacuum deposition is used as sourceand drain electrodes. In this case, the mobility is low. This is becauseSchottky barrier junction is formed at a plane where the gold having arelatively high work function and the n-type oxide semiconductor are incontact, and thus contact resistance becomes large.

Other than above, Comparative Example 2 shows the results of thetransistor using the n-type oxide semiconductor with electron carrierdensity of 4.0×10¹⁷ cm⁻³ or greater, and gold formed by vacuumdeposition is used as source and drain electrode. In the case where theelectron carrier density is in the range of 4.0×10¹⁷ cm⁻³ or greater butless than or equal to 2.0×10¹⁸ cm⁻³, transistor properties that thefield-effect mobility is high, an on/off ratio is high, and an absolutevalue of the turn-on voltage is small are obtained. However, the turn-onvoltage is negative, that means it is not normally-off operation. Whenthe electron carrier density is equal to or greater than theaforementioned range (the electron carrier density is 5.2×10¹⁸ cm⁻³ orgreater), the on/off ratio tends to be smaller than those of Examples 7to 12, and the absolute value of the turn-on voltage tends to be large.This tendency becomes significant with the electron carrier density of1.0×10¹⁹ cm⁻³ or greater.

Example 13

A bottom gate/top contact field-effect transistor was produced in thesame manner as in Example 1, provided that the formation of the activelayer was changed to the following method. Moreover, the evaluationswere carried out in the same manner as in Example 1. The results aredepicted in Table 15.

—Formation of Active Layer—

On the formed insulating layer, a Zn—Sn based oxide thin film was formedby radio frequency sputtering. As for a target, a polycrystallinesintered compact (size: 4 inches in diameter) having a composition ofZn₂SnO₄ was used. The back pressure in the sputtering chamber was set to2×10⁻⁵ Pa. An amount of oxygen in the oxide semiconductor film could becontrolled by adjusting flow rates of argon gas and oxygen gas suppliedduring the sputtering, to thereby control the electron carrier density.The total pressure was set to 0.3 Pa.

During the sputtering, the temperature of the substrate was controlledin the region of 15° C. to 35° C. by cooling a holder holding thesubstrate with water. The sputtering power was set to 150 W, and thesputtering duration was set to 20 minutes, to thereby form a Zn—Sn basedoxide film having a thickness of 50 nm. The oxygen flow rate during thesputtering is depicted in Table 15.

TABLE 15 Film coating condition, Carrier oxygen Carrier Source- FilmWork mobility On/off Turn-on flow rate density drain forming functionμFE ratio voltage (vol %) n (cm⁻³) electrode method (eV) (cm²/Vs) (×10⁸)(V) Ex. 13 50 7.5E+18 Au Coating 4.91 7.3 5.8 0.5

Example 14

A bottom gate/top contact field-effect transistor was produced in thesame manner as in Example 1, provided that the formation of the activelayer was changed to the following method. Moreover, the evaluationswere carried out in the same manner as in Example 1. The results aredepicted in Table 16.

—Formation of Active Layer—

On the formed insulating layer, a Zn—Ti based oxide thin film was formedby radio frequency sputtering. As for a target, a polycrystallinesintered compact (size: 4 inches in diameter) having a composition ofZn₂TiO₄ was used. The back pressure in the sputtering chamber was set to2×10⁻⁵ Pa. An amount of oxygen in the oxide semiconductor film could becontrolled by adjusting flow rates of argon gas and oxygen gas suppliedduring the sputtering, to thereby control the electron carrier density.The total pressure was set to 0.3 Pa.

During the sputtering, the temperature of the substrate was controlledin the region of 15° C. to 35° C. by cooling a holder holding thesubstrate with water. The sputtering power was set to 140 W, and thesputtering duration was set to 25 minutes, to thereby form a Zn—Ti basedoxide film having a thickness of 50 nm. The oxygen flow rate during thesputtering is depicted in Table 16.

TABLE 16 Film coating condition, Carrier oxygen Carrier Source- FilmWork mobility On/off Turn-on flow rate density drain forming functionμFE ratio voltage (vol %) n (cm⁻³) electrode method (eV) (cm²/Vs) (×10⁸)(V) Ex. 14 50 8.3E+18 Au Coating 4.91 7.6 4.8 1.0

Examples 15 to 17

Each bottom gate/top contact field-effect transistor was produced in thesame manner as in Example 1, provided that the formation of the activelayer and the production of the element for hall effect measurement werechanged to the following methods. Moreover, the evaluations were carriedout in the same manner as in Example 1. The results are depicted inTable 18.

—Formation of Active Layer—

In a beaker, 3.55 g of indium nitrate (In(NO₃)₃.3H₂O) and 0.139 g ofstrontium chloride (SrCl₂.6H₂O) were weighed. To this, 20 mL of1,2-propanediol, and 20 mL of ethylene glycol monomethyl ether wasadded, and the resultant was mixed at room temperature to dissolve, tothereby produce Coating Liquid 1 for forming an n-type oxidesemiconductor film used in Example 15.

In the similar manner, 3.55 g of indium nitrate (In(NO₃)₃.3H₂O) and0.125 g of calcium nitrate (Ca(NO₃)₂.4H₂O) were weighed in a beaker. Tothis, 20 mL of 1,2-propanediol and 20 mL of ethylene glycol monomethylether were added, and the resultant was mixed at room temperature todissolve, to thereby produce Coating Liquid 2 for forming an n-typeoxide semiconductor film used in Example 16.

In the similar manner, 3.55 g of indium nitrate (In(NO₃)₃.3H₂O) and0.125 g of barium chloride (BaCl₂.2H₂O) were weighed in a beaker. Tothis, 20 mL of 1,2-ethanediol, and 20 mL of ethylene glycol monomethylether were added, and the resultant was mixed at room temperature todissolve, to thereby produce Coating Liquid 3 for forming an n-typeoxide semiconductor film used in Example 17.

On the formed insulating layer, each of Coating Liquids 1 to 3 forforming a n-type oxide semiconductor film was applied into thepredetermined pattern by means of an inkjet device. After drying thesubstrate for 10 minutes on the hot plate heated at 120° C., the appliedcoating liquid was baked in the air for 1 hour at 400° C., to therebyform an In—Sr based oxide film, an In—Ca based oxide film, and an In—Babased oxide film, respectively. The formed films were each used as anactive layer.

The composition of the coating liquid for forming a n-type oxidesemiconductor film is depicted in Table 17.

TABLE 17 Raw Raw Raw Raw Active material A material B material Cmaterial D layer Type (g) Type (g) Type (mL) Type (mL) Ex. 15 In—SrIndium 3.55 Strontium 0.139 1,2- 20 Ethylene 20 based nitrate chloridepropane glycol oxide diol monomethyl ether Ex. 16 In—Ca Indium 3.55Calcium 0.125 1,2- 20 Ethylene 20 based nitrate nitrate propane glycoloxide diol monomethyl ether Ex. 17 In—Ba Indium 3.55 Barium 0.125 1,2-20 Ethylene 20 based nitrate chloride ethane glycol oxide diolmonomethyl ether<Production of Element for Hall Effect Measurement>—Formation of n-Type Oxide Semiconductor Film—

In the same manner as in the aforementioned formation of the activelayer of the field-effect transistor, each of Coating Liquids 1 to 3 forforming a n-type oxide semiconductor film was applied on a glasssubstrate into a square pattern having a side of 8 mm by means of aninkjet device. After heating the substrate for 10 minutes on the hotplate heated at 120° C., the applied coating liquid was baked in the airfor 1 hour at 400° C., to thereby form an In—Sr based oxide film, anIn—Ca based oxide film, and an In—Ba based oxide film, respectively.

—Formation of Contact Electrode—

A contact electrode for hall effect measurement was formed on the glasssubstrate, on which the In—Sr based oxide film, the In—Ca based oxidefilm, or the In—Ba based oxide had been formed, by vacuum depositionusing a shadow mask. As for a deposition source, Al was used.

TABLE 18 Carrier Carrier Source- Film Work mobility On/off Turn-ondensity drain forming function μFE ratio voltage n (cm⁻³) electrodemethod (eV) (cm²/Vs) (×10⁸) (V) Ex. 15 6.1E+18 Au Coating 4.91 6.3 4.80.5 Ex. 16 5.4E+18 Au Coating 4.91 5.5 2.5 1.0 Ex. 17 5.1E+18 Au Coating4.91 4.9 6.8 0.5

Tables 15 to 16 depict the film forming conditions of the oxidesemiconductor films formed in Examples 13 to 14, the carrier density ofthe oxide semiconductor films, types of source and drain electrodes, theforming methods of the source and drain electrode, the work functions ofthe metal films formed in the same manner as the source and drainelectrodes, and the transistor characteristics of the field-effecttransistors. Table 18 depict the carrier density of the oxidesemiconductor films formed in Examples 15 to 17, types of source anddrain electrodes, the forming methods of the source and drain electrode,the work functions of the metal films formed in the same manner as thesource and drain electrodes, and the transistor characteristics of thefield-effect transistors.

Examples 13 to 17 show the results when Au formed by coating is used assource and drain electrode. From the transistor characteristics depictedin Tables 15 to 16 and 18, it could be understood that the normally-offtransistor operation of a high field-effect mobility, a high on/offratio, and a small absolute value of the turn-on voltage was attainedwith the electron carrier density in the range of 5.1×10¹⁸ cm⁻³ to8.3×10¹⁸ cm⁻³. Since the electron carrier density of the oxidesemiconductor was high, contact resistance between the metal and theoxide semiconductor was not large at the contact interface, andtherefore the desirable device properties were exhibited.

Example 18 Production of Field-Effect Transistor

—Formation of Gate Electrode—

On a glass substrate, Al was deposited to have a thickness of 100 nmthrough vapor deposition, followed by patterning into a line throughphotolithography and etching, to thereby form a gate electrode.

—Formation of Gate Insulating Layer—

Next, a film of SiO₂ was formed to have a thickness of 200 nm by plasmaCVD, to thereby form a gate insulating film.

—Formation of Active Layer—

On the formed insulating layer, a Mg—In based oxide semiconductor film(active layer) was formed by sputtering in the method described inExamples of JP-A No. 2010-74148. As for a target, a polycrystallinesintered compact having a composition of In₂MgO₄ was used. The backpressure in the sputtering chamber was set to 2×10⁻⁵ Pa. The totalpressure was set to 0.3 Pa by adjusting flow rates of argon gas andoxygen gas supplied during the sputtering. An amount of oxygen in theoxide semiconductor film and the electron carrier density werecontrolled by adjusting the flow rate of the oxygen gas. The oxygen flowrate during the sputtering is depicted in Table 19.

A thickness of the obtained oxide semiconductor film (active layer) was50 nm.

—Formation of Source Electrode and Drain Electrode—

On the formed active layer, a gold nanometal ink (Au-1Chb, manufacturedby ULVAC, Inc., the average particle diameter: 5 nm, the metal content:50% by mass) was applied into the predetermined pattern by means of aninkjet device. The applied ink was heated in the air for 30 minutes at300° C., to form a source electrode and a drain electrode each having athickness of 100 nm. A channel width was 400 μm, and a channel lengthspecified with a length between the source electrode and the drainelectrode was 50 μm.

In the manner as described above, a bottom gate/top contact field-effecttransistor was produced.

—Production of Element Used for Work Function Measurement—

In order to measure work function, an element used for work functionmeasurement was obtained by forming a metal film on a glass substrate inthe same manner as in the formation of the source electrode and drainelectrode.

<Production of Element for Hall Effect Measurement>

—Formation of n-Type Oxide Semiconductor Film—

In the same manner as in the aforementioned formation of the activelayer of the field-effect transistor, a square pattern of a n-type oxidesemiconductor having a side of 8 mm was formed on a glass substratethrough a shadow mask. The conditions for the sputtering were set thesame as in the formation of the active layer. The conditions forsputtering are depicted in Table 19.

—Formation of Contact Electrode—

A contact electrode for hall effect measurement was formed on the glasssubstrate, on which the n-type oxide semiconductor film had been formed,by vacuum deposition using a shadow mask. As for a deposition source, Alwas used.

<Evaluation of Electron Carrier Density (Concentration)>

The element for hall effect measurement was subjected to themeasurements of specific resistance and hall effect by means of a halleffect measurement system (ResiTest8300, manufactured by TOYOCorporation), to thereby determined an electron carrier density of(cm⁻³) the n-type oxide semiconductor. The obtained electron carrierdensity (concentration) is depicted in Table 19.

<Evaluation of Work Function>

The work function of the metal film of the element for work functionmeasurement was determined by means of a photoelectron spectrometer inair AC-2 (manufactured by RIKEN KEIKI Co., Ltd.). The obtained workfunction is depicted in Table 19.

<Evaluation of Transistor Performance>

An evaluation of transistor performance was performed on the obtainedfield-effect transistor by means of a semiconductor parameter analyzer(Semiconductor Parameter Analyzer 4156C, manufactured by AgilentTechnologies, Inc.). Current-voltage characteristics were evaluated bysetting the source-drain voltage Vds to 20 V, and varying the gatevoltage Vg from −30 V to +30 V. FIG. 21 depicts the measuredcurrent-voltage characteristics (transmission characteristics). Thefield-effect mobility was calculated in the saturated region. Moreover,a ratio (on/off ratio) of the source-drain current Ids of the transistorin the “on” state (e.g., Vg=20 V) to that in the “off” state (e.g.,Vg=−20 V) was calculated. Moreover, the gate voltage (turn-on voltage),with which the source-drain current Ids turned to increase wascalculated. The results are depicted in Table 19.

Moreover, the current-voltage characteristics (output characteristics)were evaluated by varying the gate voltage Vg from 0 V to +30 V stepwiseby 5 V, and varying the source-drain voltage Vds from 0V to +20 V ateach gate voltage. FIG. 22 depicts the measured current-voltagecharacteristics (output characteristics).

Comparative Example 3

A bottom gate/top contact field-effect transistor was produced in thesame manner as in Example 18, provided that the formation of the activelayer, and the formation of the source electrode and drain electrodewere changed to the following methods. Moreover, the evaluations werecarried out in the same manner as in Example 18. The results aredepicted in Table 19. Moreover, FIG. 23 depicts the current-voltagecharacteristics (transmission characteristics) thereof, and FIG. 24depicts the current-voltage characteristics (output characteristics)thereof.

—Formation of Active Layer—

On the formed source electrode and drain electrode as well as the gateinsulating layer, a Mg—In based oxide semiconductor film (active layer)was formed by sputtering in the method described in JP-A No. 2010-74148.As for a target, a polycrystalline sintered compact having a compositionof In₂MgO₄. The back pressure in the sputtering chamber was set to2×10⁻⁵ Pa. The total pressure was set to 0.3 Pa by adjusting flow ratesof argon gas and oxygen gas supplied during the sputtering. An amount ofoxygen in the oxide semiconductor film and the electron carrier densitywere controlled by adjusting the flow rate of the oxygen gas. The oxygenflow rate during the sputtering is depicted in Table 19.

A thickness of the obtained oxide semiconductor film (active layer) was50 nm.

—Formation of Source Electrode and Drain Electrode—

On the formed active layer, a source electrode and a drain electrodeeach having a thickness of 100 nm were formed by vacuum deposition. Asfor the deposition source, Al was used. A channel width was 400 μm, anda channel length was 50 μm.

TABLE 19 Film coating condition, Carrier oxygen Carrier Source- FilmWork mobility On/off Turn-on flow rate density drain forming functionμFE ratio voltage (vol %) n (cm⁻³) electrode method (eV) (cm²/Vs) (×10⁸)(V) Ex. 18 0.80 5.2E+18 Au Coating 4.91 6.4 1.5 0.5 Comp. 50 5.7E+15 AlVacuum 4.30 2.3 0.61 −2.0 Ex. 3 deposition

Table 19 depicts the film forming conditions of the oxide semiconductorfilms formed in Example 18 and Comparative Example 3, the carrierdensity of the oxide semiconductor films, types of source and drainelectrodes, the forming methods of the source and drain electrode, thework functions of the metal films formed in the same manner as thesource and drain electrodes, and the transistor characteristics of thefield-effect transistors.

It can be confirmed from the current-voltage characteristics (outputcharacteristics) depicted in FIG. 22 that the value of the source-draincurrent is linearly increased in the region where the value of thesource-drain voltage is small. It can be assumed from this result thatthe contact interface between the metal and oxide semiconductor is anohmic contact.

From the current-voltage characteristics (transmission characteristics)depicted in FIG. 21, and the results of the transistor characteristicsof Example 18 depicted in Table 19, it was found that, when the electroncarrier density was 5.2×10¹⁸ cm⁻³, normally off transistorcharacteristics where the field-effect mobility was high, the on/offratio was high, and the absolute value of the turn-on voltage was smallwas obtained.

Since the electron carrier density of the oxide semiconductor was high,contact resistance between the metal and the oxide semiconductor was notlarge at the contact interface, and therefore the desirable deviceproperties were exhibited.

Use of the transistor having the current-voltage characteristics (outputcharacteristics) as in FIG. 22 as a driving circuit is preferable,because designing of driving voltage becomes easy.

On the other hand, it could be confirmed from the current-voltagecharacteristics (output characteristics) of Comparative Example 3depicted in FIG. 24 that the source-drain current was non-linearlyincreased in the shape of a concave in the region where the value of thesource-drain voltage was small.

In the case where the electron carrier density of the oxidesemiconductor is 5.7×10¹⁵ cm⁻³, as depicted in Table 19 and FIG. 23, thetransistor characteristics that the field-effect mobility is high, theon/off ratio is high, and the absolute value of the turn-on voltage islow can be attained by using Al, which is a metal having low workfunction, as source and drain electrodes.

However, FIG. 24 shows that the contact resistance at the contactinterface between the metal and the oxide semiconductor depends on thevoltage, and it is assumed that the contact interface of the metal andthe oxide semiconductor is a non-ohmic contact. When the transistorhaving the current-voltage characteristics as depicted in FIG. 24 isused as a driving circuit, current is largely changed with a slightchange in the voltage applied to the transistor, and therefore it isdifficult to apply the transistor as the driving circuit. By using ametal having low work function for source and drain electrode, andcontrolling the electron carrier density of oxide semiconductor in therange of 1×10¹⁵ cm⁻³ to 1×10¹⁶ cm⁻³, the transistor characteristics thatthe field-effect mobility is high, the on/off ratio is high, and theabsolute value of the turn-on voltage is small can be attained. On theother hand, however, the contact interface between the metal and oxidesemiconductor becomes a non-ohmic contact, and therefore it is difficultto apply to a driving circuit.

It can be understood from the results of Example 18 and ComparativeExample 3 that the field-effect transistor of the present invention canbe easily applied as a driving circuit of a display element, compared toa conventional field-effect transistor using oxide semiconductor havingthe electron carrier density (concentration) of 1×10¹⁵ cm⁻³ to 1×10¹⁶cm⁻³, which is typically suitable used as a n-type oxide semiconductorof a transistor, and a metal having low work function for source anddrain electrodes.

The embodiments of the present invention are, for example, as follows:

<1> A field-effect transistor, containing:

a gate electrode configured to apply gate voltage;

a source electrode and a drain electrode, both of which are configuredto take out electric current;

an active layer formed of a n-type oxide semiconductor, provided incontact with the source electrode and the drain electrode; and

a gate insulating layer provided between the gate electrode and theactive layer,

wherein work function of the source electrode and drain electrode is4.90 eV or greater, and

wherein an electron carrier density of the n-type oxide semiconductor is4.0×10¹⁷ cm⁻³ or greater.

<2> The field-effect transistor according to <1>, wherein a material ofthe source electrode and the drain electrode is a metal, an alloy, orboth.

<3> The field-effect transistor according to <2>, wherein the metal, thealloy, or both contain gold, silver, palladium, platinum, nickel,iridium, rhodium, or any combination thereof.

<4> The field-effect transistor according to any one of <1> to <3>,wherein the source electrode and the drain electrode are formed bybaking metal particles, alloy particles, an organometallic compound, orany combination thereof.

<5> The field-effect transistor according to <4>, wherein the sourceelectrode and the drain electrode are formed by applying a coatingliquid containing metal particles, alloy particles, an organometalliccompound, or any combination thereof in a droplet ejecting system, andbaking the metal particles, the alloy particles, the organometalliccompound, or any combination thereof.<6> The field-effect transistor according to any one of <1> to <5>,wherein the n-type oxide semiconductor contains indium, zinc, tin,gallium, titanium, or any combination thereof.<7> The field-effect transistor according to <6>, wherein the n-typeoxide semiconductor further contains an alkaline earth metal.<8> A method for producing the field-effect transistor according to anyone of <1> to <7>, the method containing:

applying a coating liquid containing metal particles, alloy particles,an organometallic compound, or any combination thereof at least on thegate insulating layer, and baking the metal particles, the alloyparticles, the organometallic compound, or any combination thereof toform the source electrode and the drain electrode.

<9> A method for producing the field-effect transistor according to anyone of <1> to <7>, the method containing:

applying a coating liquid containing metal particles, alloy particles,an organometallic compound, or any combination thereof at least on abase, and baking the metal particles, the alloy particles, theorganometallic compound, or any combination thereof to form the sourceelectrode and the drain electrode.

<10> The method according to any of <8> or <9>, wherein the metalparticles, the alloy particles, the organometallic compound, or anycombination thereof contain gold, silver, palladium, platinum, nickel,iridium, rhodium, or any combination thereof.

<11> A display element, containing:

a light control element configured to control light output according toa driving signal; and

a driving circuit, which contains the field-effect transistor accordingto any one of <1> to <7>, and is configured to drive the light controlelement.

<12> The display element according to <11>, wherein the light controlelement contains an electroluminescent element or an electrochromicelement.

<13> The display element according to <11>, wherein the light controlelement contains a liquid crystal element, or an electrophoreticelement.

<14> An image display device, which displays an image corresponding toimage data, and which containing:

a plurality of the display elements according to any one of <11> to<13>, arranged in a matrix;

a plurality of lines configured to separately apply gate voltage tofield-effect transistors in each of the display elements; and

a display control device configured to individually control the gatevoltage of each of the field-effect transistors through the linescorresponding to the image data.

<15> A system, containing:

the image display device according to <14>; and

an image data generating device configured to generate an image databased on image information to be displayed, and to output the generatedimage data to the image display device.

REFERENCE SIGNS LIST

-   -   1 base    -   2 gate electrode    -   3 gate insulating layer    -   4 source electrode    -   5 drain electrode    -   6 active layer    -   10 field-effect transistor    -   20 field-effect transistor    -   21 base    -   22 active layer    -   23 source electrode    -   24 drain electrode    -   25 gate insulating layer    -   26 gate electrode    -   30 capacitor    -   40 field-effect transistor    -   100 television device    -   101 main control device    -   103 tuner    -   104 AD converter (ADC)    -   105 demodulating circuit    -   106 TS (Transport Stream) decoder    -   111 sound decoder    -   112 DA converter (DAC)    -   113 sound output circuit    -   114 speaker    -   121 image decoder    -   122 image-OSD synthesis circuit    -   123 image output circuit    -   124 image display device    -   125 OSD drawing circuit    -   131 memory    -   132 operating device    -   141 drive interface (drive IF)    -   142 hard disk device    -   143 optical disk device    -   151 IR photodetector    -   152 communication control unit    -   210 aerial    -   220 remote-controlled transmitter    -   300 display unit    -   302, 302′ display element    -   310 display    -   312 cathode    -   314 anode    -   320, 320′ driving circuit    -   340 organic EL thin film layer    -   342 electron transporting layer    -   344 light emitting layer    -   346 hole transporting layer    -   350 organic EL element    -   360 interlayer insulating film    -   361 capacitor    -   370 liquid crystal element    -   400 display control device    -   402 image data processing circuit    -   404 scanning line driving circuit    -   406 data line driving circuit

The invention claimed is:
 1. A field-effect transistor, comprising: agate electrode; a source electrode and a drain electrode, each formed bya conductive material; a n-type oxide semiconductor, provided in contactwith the source electrode and the drain electrode; and a gate insulatinglayer provided between the gate electrode and the n-type oxidesemiconductor, wherein a work function of the source electrode and thedrain electrode formed by the conductive material is 4.90 eV or greater,and, in a case that an electron carrier density of the n-type oxidesemiconductor is 4.0×10¹⁸ cm⁻³ or greater, a carrier mobility of thefield-effect transistor is at least 3.8 cm²/Vs and an ON/OFF ratio of adrain-source current is 1.0×10⁸ or greater between an OFF state in whicha gate voltage is −20 V and an ON state in which the gate voltage is 20V.
 2. The field-effect transistor according to claim 1, wherein theconductive material of the source electrode and the drain electrode is ametal, an alloy, or both.
 3. The field-effect transistor according toclaim 2, wherein the metal, the alloy, or both contain gold, silver,palladium, platinum, nickel, iridium, rhodium, or any combinationthereof.
 4. The field-effect transistor according to claim 1, whereinthe source electrode and the drain electrode are formed by baking metalparticles, alloy particles, an organometallic compound, or anycombination thereof.
 5. The field-effect transistor according to claim1, wherein the n-type oxide semiconductor contains indium, zinc, tin,gallium, titanium, or any combination thereof.
 6. The field-effecttransistor according to claim 5, wherein the n-type oxide semiconductorfurther contains an alkaline earth metal.
 7. A display element,comprising: a light control element configured to control light outputaccording to a driving signal; and a driving circuit, which contains thefield-effect transistor according to claim 1 and is configured to drivethe light control element.
 8. The display element according to claim 7,wherein the light control element contains an electroluminescent elementor an electrochromic element.
 9. The display element according to claim7, wherein the light control element contains a liquid crystal elementor an electrophoretic element.
 10. An image display device, whichdisplays an image corresponding to image data, the image display devicecomprising: a plurality of display elements according to claim 7arranged in a matrix; a plurality of lines configured to separatelyapply a respective gate voltage to field-effect transistors in each ofthe plurality of display elements; and a display control deviceconfigured to individually control the respective gate voltage of eachof the field-effect transistors through the lines corresponding to theimage data.
 11. A system, comprising: the image display device accordingto claim 10; and an image data generating device configured to generateimage data based on image information to be displayed, and output thegenerated image data to the image display device.
 12. The field-effecttransistor according to claim 1, wherein each of the source electrodeand the drain electrode is a thin film electrode formed by theconductive material, and the work function of the thin film electrodeformed by the conductive material is 4.90 eV or greater.
 13. Thefield-effect transistor according to claim 1, wherein each of the sourceelectrode and the drain electrode is a thin film electrode formed by theconductive material, and the work function of the thin film electrodeformed by the conductive material is 4.90 eV or greater and is 6.00 eVor less.
 14. The field-effect transistor according to claim 1, whereineach of the source electrode and the drain electrode is a thin filmelectrode formed by the conductive material, and the work function ofthe thin film electrode formed by the conductive material is 4.90 eV orgreater, to prevent the thin film electrode from being oxidized by aheat treatment or oxidizing atmosphere treatment performed after thefield-effect transistor has been formed.